Fair Value Measurements - Schedule of Fair Value of Financial Assets (Details) - USD ($) $ in Thousands | Mar. 31, 2021 | Dec. 31, 2020 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable Securities, Amortized Cost | $ 2,183,707 | $ 1,978,283 |
Marketable Securities, Unrealized Gains | 1,186 | 1,700 |
Marketable Securities, Unrealized Losses | (563) | (334) |
Marketable Securities, Fair Value | 2,184,330 | 1,979,649 |
Total Financial Assets, Amortized Cost | 2,567,288 | 2,421,372 |
Total Financial Assets, Fair Value | 2,567,911 | 2,422,738 |
Level I | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable Securities, Fair Value | 661,518 | 523,506 |
Total Financial Assets, Fair Value | 1,045,099 | 966,595 |
Level II | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable Securities, Fair Value | 1,522,812 | 1,456,143 |
Total Financial Assets, Fair Value | 1,522,812 | 1,456,143 |
Level III | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable Securities, Fair Value | 0 | 0 |
Total Financial Assets, Fair Value | 0 | 0 |
Money market funds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash Equivalents, Amortized Cost | 379,345 | 438,854 |
Cash Equivalents, Fair Value | 379,345 | 438,854 |
Money market funds | Level I | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash Equivalents, Fair Value | 379,345 | 438,854 |
Money market funds | Level II | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash Equivalents, Fair Value | 0 | 0 |
Money market funds | Level III | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash Equivalents, Fair Value | 0 | 0 |
Commercial paper | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable Securities, Amortized Cost | 46,658 | 51,211 |
Marketable Securities, Unrealized Gains | 0 | 0 |
Marketable Securities, Unrealized Losses | 0 | 0 |
Marketable Securities, Fair Value | 46,658 | 51,211 |
Commercial paper | Level I | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable Securities, Fair Value | 0 | 0 |
Commercial paper | Level II | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable Securities, Fair Value | 46,658 | 51,211 |
Commercial paper | Level III | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable Securities, Fair Value | 0 | 0 |
Certificate of deposits | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable Securities, Amortized Cost | 80,069 | 50,136 |
Marketable Securities, Unrealized Gains | 0 | 3 |
Marketable Securities, Unrealized Losses | 0 | 0 |
Marketable Securities, Fair Value | 80,069 | 50,139 |
Certificate of deposits | Level I | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable Securities, Fair Value | 0 | 0 |
Certificate of deposits | Level II | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable Securities, Fair Value | 80,069 | 50,139 |
Certificate of deposits | Level III | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable Securities, Fair Value | 0 | 0 |
U.S. government notes | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable Securities, Amortized Cost | 661,366 | 523,320 |
Marketable Securities, Unrealized Gains | 190 | 187 |
Marketable Securities, Unrealized Losses | (38) | (1) |
Marketable Securities, Fair Value | 661,518 | 523,506 |
U.S. government notes | Level I | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable Securities, Fair Value | 661,518 | 523,506 |
U.S. government notes | Level II | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable Securities, Fair Value | 0 | 0 |
U.S. government notes | Level III | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable Securities, Fair Value | 0 | 0 |
Corporate bonds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable Securities, Amortized Cost | 972,063 | 878,484 |
Marketable Securities, Unrealized Gains | 709 | 1,167 |
Marketable Securities, Unrealized Losses | (522) | (330) |
Marketable Securities, Fair Value | 972,250 | 879,321 |
Corporate bonds | Level I | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable Securities, Fair Value | 0 | 0 |
Corporate bonds | Level II | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable Securities, Fair Value | 972,250 | 879,321 |
Corporate bonds | Level III | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable Securities, Fair Value | 0 | 0 |
Agency securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable Securities, Amortized Cost | 423,551 | 475,132 |
Marketable Securities, Unrealized Gains | 287 | 343 |
Marketable Securities, Unrealized Losses | (3) | (3) |
Marketable Securities, Fair Value | 423,835 | 475,472 |
Agency securities | Level I | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable Securities, Fair Value | 0 | 0 |
Agency securities | Level II | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable Securities, Fair Value | 423,835 | 475,472 |
Agency securities | Level III | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable Securities, Fair Value | 0 | 0 |
Money market funds - restricted | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other Assets, Amortized Cost | 4,236 | 4,235 |
Other Assets, Fair Value | 4,236 | 4,235 |
Money market funds - restricted | Level I | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other Assets, Fair Value | 4,236 | 4,235 |
Money market funds - restricted | Level II | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other Assets, Fair Value | 0 | 0 |
Money market funds - restricted | Level III | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other Assets, Fair Value | $ 0 | $ 0 |