Fair Value Measurements - Financial Assets and Liabilities Measured on a Recurring and Nonrecurring Basis (Details) - USD ($) $ in Thousands | Sep. 30, 2023 | Dec. 31, 2022 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | $ 36,201 | $ 36,758 |
Investment securities | 52,623 | 52,610 |
Fair Value, Recurring | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | | 13,639 |
Securities owned — trading | 36,201 | 36,758 |
Other assets | 744,024 | 617,477 |
Total assets at fair value | 1,376,925 | 1,798,914 |
Accounts payable and accrued liabilities | | 3,860 |
Other Liabilities, Fair Value Disclosure | 197,295 | 122,338 |
Total liabilities at fair value | 197,295 | 126,198 |
Fair Value, Recurring | Fair Value, Inputs, Level 1 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | | 13,639 |
Securities owned — trading | 36,162 | 36,173 |
Other assets | 739,806 | 612,229 |
Total assets at fair value | 1,372,668 | 1,793,081 |
Accounts payable and accrued liabilities | | 0 |
Other Liabilities, Fair Value Disclosure | 147,908 | 122,277 |
Total liabilities at fair value | 147,908 | 122,277 |
Fair Value, Recurring | Fair Value, Inputs, Level 2 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | | 0 |
Securities owned — trading | 39 | 585 |
Other assets | 4,218 | 5,248 |
Total assets at fair value | 4,257 | 5,833 |
Securities sold, but not yet purchased | | 0 |
Accounts payable and accrued liabilities | | 0 |
Other Liabilities, Fair Value Disclosure | 0 | 61 |
Total liabilities at fair value | 0 | 61 |
Fair Value, Recurring | Fair Value, Inputs, Level 3 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | | 0 |
Securities owned — trading | 0 | 0 |
Other assets | 0 | 0 |
Total assets at fair value | 0 | 0 |
Securities sold, but not yet purchased | 0 | 0 |
Accounts payable and accrued liabilities | | 3,860 |
Other Liabilities, Fair Value Disclosure | 49,387 | 0 |
Total liabilities at fair value | 49,387 | 3,860 |
Equity Securities [Member] | Fair Value, Recurring | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities sold, but not yet purchased | 44 | 20 |
Equity Securities [Member] | Fair Value, Recurring | Fair Value, Inputs, Level 1 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities sold, but not yet purchased | 44 | 20 |
Equity Securities [Member] | Fair Value, Recurring | Fair Value, Inputs, Level 2 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities sold, but not yet purchased | 0 | 0 |
Equity Securities [Member] | Fair Value, Recurring | Fair Value, Inputs, Level 3 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities sold, but not yet purchased | 0 | 0 |
Debt Securities [Member] | Fair Value, Recurring | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities sold, but not yet purchased | | 61 |
Debt Securities [Member] | Fair Value, Recurring | Fair Value, Inputs, Level 1 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities sold, but not yet purchased | | 0 |
Debt Securities [Member] | Fair Value, Recurring | Fair Value, Inputs, Level 2 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities sold, but not yet purchased | | 61 |
Debt Securities [Member] | Fair Value, Recurring | Fair Value, Inputs, Level 3 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities sold, but not yet purchased | | 0 |
Mutual Funds [Member] | Fair Value, Recurring | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities sold, but not yet purchased | | 4 |
Mutual Funds [Member] | Fair Value, Recurring | Fair Value, Inputs, Level 1 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities sold, but not yet purchased | | 4 |
Mutual Funds [Member] | Fair Value, Recurring | Fair Value, Inputs, Level 2 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities sold, but not yet purchased | | 0 |
Mutual Funds [Member] | Fair Value, Recurring | Fair Value, Inputs, Level 3 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities sold, but not yet purchased | | 0 |
Securities Sold, Not yet Purchased | Fair Value, Recurring | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities sold, but not yet purchased | 44 | 85 |
Securities Sold, Not yet Purchased | Fair Value, Recurring | Fair Value, Inputs, Level 1 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities sold, but not yet purchased | 44 | 24 |
Securities Sold, Not yet Purchased | Fair Value, Recurring | Fair Value, Inputs, Level 2 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities sold, but not yet purchased | 0 | 61 |
Securities Sold, Not yet Purchased | Fair Value, Recurring | Fair Value, Inputs, Level 3 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities sold, but not yet purchased | | 0 |
Cash Equivalents Segregated Under Federal Or Other Regulations | Fair Value, Recurring | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | 596,034 | 1,131,040 |
Cash Equivalents Segregated Under Federal Or Other Regulations | Fair Value, Recurring | Fair Value, Inputs, Level 1 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | 596,034 | 1,131,040 |
Cash Equivalents Segregated Under Federal Or Other Regulations | Fair Value, Recurring | Fair Value, Inputs, Level 2 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | 0 | 0 |
Cash Equivalents Segregated Under Federal Or Other Regulations | Fair Value, Recurring | Fair Value, Inputs, Level 3 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | 0 | 0 |
Cash Equivalents Not Subject To Segregation | Fair Value, Recurring | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | 666 | |
Cash Equivalents Not Subject To Segregation | Fair Value, Recurring | Fair Value, Inputs, Level 1 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | 666 | |
Cash Equivalents Not Subject To Segregation | Fair Value, Recurring | Fair Value, Inputs, Level 2 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | 0 | |
Cash Equivalents Not Subject To Segregation | Fair Value, Recurring | Fair Value, Inputs, Level 3 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | 0 | |
Money Market Funds [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 107 | 112 |
Money Market Funds [Member] | Fair Value, Recurring | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 107 | 112 |
Money Market Funds [Member] | Fair Value, Recurring | Fair Value, Inputs, Level 1 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 107 | 112 |
Money Market Funds [Member] | Fair Value, Recurring | Fair Value, Inputs, Level 2 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 0 | 0 |
Money Market Funds [Member] | Fair Value, Recurring | Fair Value, Inputs, Level 3 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 0 | 0 |
Other Security Investments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 39 | 585 |
Other Security Investments | Fair Value, Recurring | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 39 | 585 |
Other Security Investments | Fair Value, Recurring | Fair Value, Inputs, Level 1 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 0 | 0 |
Other Security Investments | Fair Value, Recurring | Fair Value, Inputs, Level 2 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 39 | 585 |
Other Security Investments | Fair Value, Recurring | Fair Value, Inputs, Level 3 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 0 | 0 |
Mutual Funds [Member] | Fair Value, Recurring | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 10,917 | 10,679 |
Mutual Funds [Member] | Fair Value, Recurring | Fair Value, Inputs, Level 1 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 10,917 | 10,679 |
Mutual Funds [Member] | Fair Value, Recurring | Fair Value, Inputs, Level 2 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 0 | 0 |
Mutual Funds [Member] | Fair Value, Recurring | Fair Value, Inputs, Level 3 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 0 | 0 |
Equity Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 97 | 980 |
Equity Securities [Member] | Fair Value, Recurring | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 97 | 980 |
Equity Securities [Member] | Fair Value, Recurring | Fair Value, Inputs, Level 1 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 97 | 980 |
Equity Securities [Member] | Fair Value, Recurring | Fair Value, Inputs, Level 2 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 0 | 0 |
Equity Securities [Member] | Fair Value, Recurring | Fair Value, Inputs, Level 3 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 0 | 0 |
U.S. treasury obligations [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 25,041 | 24,402 |
U.S. treasury obligations [Member] | Fair Value, Recurring | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 25,041 | 24,402 |
U.S. treasury obligations [Member] | Fair Value, Recurring | Fair Value, Inputs, Level 1 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 25,041 | 24,402 |
U.S. treasury obligations [Member] | Fair Value, Recurring | Fair Value, Inputs, Level 2 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 0 | 0 |
U.S. treasury obligations [Member] | Fair Value, Recurring | Fair Value, Inputs, Level 3 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 0 | 0 |
Deferred Compensation Plan | Fair Value, Recurring | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other assets | 591,942 | 489,976 |
Deferred Compensation Plan | Fair Value, Recurring | Fair Value, Inputs, Level 1 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other assets | 591,942 | 489,976 |
Deferred Compensation Plan | Fair Value, Recurring | Fair Value, Inputs, Level 2 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other assets | 0 | 0 |
Deferred Compensation Plan | Fair Value, Recurring | Fair Value, Inputs, Level 3 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other assets | 0 | 0 |
Other Investments | Fair Value, Recurring | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other assets | 4,218 | 5,248 |
Other Investments | Fair Value, Recurring | Fair Value, Inputs, Level 1 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other assets | | 0 |
Other Investments | Fair Value, Recurring | Fair Value, Inputs, Level 2 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other assets | 4,218 | 5,248 |
Other Investments | Fair Value, Recurring | Fair Value, Inputs, Level 3 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other assets | 0 | 0 |
Fractional Shares | Fair Value, Recurring | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other assets | 147,864 | 122,253 |
Other Liabilities, Fair Value Disclosure | 147,864 | 122,253 |
Fractional Shares | Fair Value, Recurring | Fair Value, Inputs, Level 1 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other assets | 147,864 | 122,253 |
Other Liabilities, Fair Value Disclosure | 147,864 | 122,253 |
Fractional Shares | Fair Value, Recurring | Fair Value, Inputs, Level 2 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other assets | 0 | 0 |
Other Liabilities, Fair Value Disclosure | 0 | |
Fractional Shares | Fair Value, Recurring | Fair Value, Inputs, Level 3 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other assets | 0 | $ 0 |
Other Liabilities, Fair Value Disclosure | 0 | |
contingent consideration [Domain] | Fair Value, Recurring | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other Liabilities, Fair Value Disclosure | 49,387 | |
contingent consideration [Domain] | Fair Value, Recurring | Fair Value, Inputs, Level 1 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other Liabilities, Fair Value Disclosure | 0 | |
contingent consideration [Domain] | Fair Value, Recurring | Fair Value, Inputs, Level 2 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other Liabilities, Fair Value Disclosure | 0 | |
contingent consideration [Domain] | Fair Value, Recurring | Fair Value, Inputs, Level 3 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other Liabilities, Fair Value Disclosure | $ 49,387 | |