Fair Value Disclosures - Fair Value for Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions | Sep. 30, 2024 | Dec. 31, 2023 |
Financial Assets | | |
Fixed maturity securities, fair value | $ 7,596.1 | $ 6,912.1 |
Equity securities at fair value | 219.4 | 223 |
Other investments | 501.7 | 499 |
States, municipalities and political subdivisions | | |
Financial Assets | | |
Fixed maturity securities, fair value | 126.5 | 149.2 |
Foreign governments | | |
Financial Assets | | |
Fixed maturity securities, fair value | 493.5 | 479.8 |
Asset-backed | | |
Financial Assets | | |
Fixed maturity securities, fair value | 970.3 | 873.8 |
Commercial mortgage-backed | | |
Financial Assets | | |
Fixed maturity securities, fair value | 325.2 | 330.2 |
Residential mortgage-backed | | |
Financial Assets | | |
Fixed maturity securities, fair value | 624.1 | 486 |
U.S. corporate | | |
Financial Assets | | |
Fixed maturity securities, fair value | 3,433.1 | 3,130.4 |
Foreign corporate | | |
Financial Assets | | |
Fixed maturity securities, fair value | 1,564 | 1,397.5 |
Recurring | | |
Financial Assets | | |
Short-term investments | 279.9 | 210.1 |
Other investments | 66 | 62.5 |
Cash equivalents | 1,330.3 | 1,051.3 |
Other assets | 12.9 | 15.8 |
Assets held in separate accounts | 11.6 | 10.5 |
Total financial assets | 9,516.2 | 8,485.3 |
Financial Liabilities | | |
Other liabilities | 66 | 64.2 |
Liabilities related to separate accounts | 11.6 | 10.5 |
Total financial liabilities | 77.6 | 74.7 |
Recurring | Level 1 | | |
Financial Assets | | |
Short-term investments | 254.5 | 121.6 |
Other investments | 66 | 62.4 |
Cash equivalents | 1,315.1 | 1,040.4 |
Other assets | 0 | 0 |
Assets held in separate accounts | 8.7 | 6.7 |
Total financial assets | 1,661.3 | 1,264.9 |
Financial Liabilities | | |
Other liabilities | 66 | 62.4 |
Liabilities related to separate accounts | 8.7 | 6.7 |
Total financial liabilities | 74.7 | 69.1 |
Recurring | Level 2 | | |
Financial Assets | | |
Short-term investments | 25.4 | 88.5 |
Other investments | 0 | 0 |
Cash equivalents | 15.2 | 10.9 |
Other assets | 0 | 0 |
Assets held in separate accounts | 2.9 | 3.8 |
Total financial assets | 7,656.5 | 7,079 |
Financial Liabilities | | |
Other liabilities | 0 | 1.8 |
Liabilities related to separate accounts | 2.9 | 3.8 |
Total financial liabilities | 2.9 | 5.6 |
Recurring | Level 3 | | |
Financial Assets | | |
Short-term investments | 0 | 0 |
Other investments | 0 | 0.1 |
Cash equivalents | 0 | 0 |
Other assets | 12.9 | 15.8 |
Assets held in separate accounts | 0 | 0 |
Total financial assets | 198.4 | 141.4 |
Financial Liabilities | | |
Other liabilities | 0 | 0 |
Liabilities related to separate accounts | 0 | 0 |
Total financial liabilities | 0 | 0 |
Recurring | U.S. government and government agencies and authorities | | |
Financial Assets | | |
Fixed maturity securities, fair value | 59.4 | 65.2 |
Recurring | U.S. government and government agencies and authorities | Level 1 | | |
Financial Assets | | |
Fixed maturity securities, fair value | 0 | 0 |
Recurring | U.S. government and government agencies and authorities | Level 2 | | |
Financial Assets | | |
Fixed maturity securities, fair value | 59.4 | 65.2 |
Recurring | U.S. government and government agencies and authorities | Level 3 | | |
Financial Assets | | |
Fixed maturity securities, fair value | 0 | 0 |
Recurring | States, municipalities and political subdivisions | | |
Financial Assets | | |
Fixed maturity securities, fair value | 126.5 | 149.2 |
Recurring | States, municipalities and political subdivisions | Level 1 | | |
Financial Assets | | |
Fixed maturity securities, fair value | 0 | 0 |
Recurring | States, municipalities and political subdivisions | Level 2 | | |
Financial Assets | | |
Fixed maturity securities, fair value | 126.5 | 149.2 |
Recurring | States, municipalities and political subdivisions | Level 3 | | |
Financial Assets | | |
Fixed maturity securities, fair value | 0 | 0 |
Recurring | Foreign governments | | |
Financial Assets | | |
Fixed maturity securities, fair value | 493.5 | 479.8 |
Recurring | Foreign governments | Level 1 | | |
Financial Assets | | |
Fixed maturity securities, fair value | 0 | 0 |
Recurring | Foreign governments | Level 2 | | |
Financial Assets | | |
Fixed maturity securities, fair value | 493.5 | 479.8 |
Recurring | Foreign governments | Level 3 | | |
Financial Assets | | |
Fixed maturity securities, fair value | 0 | 0 |
Recurring | Asset-backed | | |
Financial Assets | | |
Fixed maturity securities, fair value | 970.3 | 873.8 |
Recurring | Asset-backed | Level 1 | | |
Financial Assets | | |
Fixed maturity securities, fair value | 0 | 0 |
Recurring | Asset-backed | Level 2 | | |
Financial Assets | | |
Fixed maturity securities, fair value | 859.9 | 791 |
Recurring | Asset-backed | Level 3 | | |
Financial Assets | | |
Fixed maturity securities, fair value | 110.4 | 82.8 |
Recurring | Commercial mortgage-backed | | |
Financial Assets | | |
Fixed maturity securities, fair value | 325.2 | 330.2 |
Recurring | Commercial mortgage-backed | Level 1 | | |
Financial Assets | | |
Fixed maturity securities, fair value | 0 | 0 |
Recurring | Commercial mortgage-backed | Level 2 | | |
Financial Assets | | |
Fixed maturity securities, fair value | 325.2 | 330.2 |
Recurring | Commercial mortgage-backed | Level 3 | | |
Financial Assets | | |
Fixed maturity securities, fair value | 0 | 0 |
Recurring | Residential mortgage-backed | | |
Financial Assets | | |
Fixed maturity securities, fair value | 624.1 | 486 |
Recurring | Residential mortgage-backed | Level 1 | | |
Financial Assets | | |
Fixed maturity securities, fair value | 0 | 0 |
Recurring | Residential mortgage-backed | Level 2 | | |
Financial Assets | | |
Fixed maturity securities, fair value | 624.1 | 486 |
Recurring | Residential mortgage-backed | Level 3 | | |
Financial Assets | | |
Fixed maturity securities, fair value | 0 | 0 |
Recurring | U.S. corporate | | |
Financial Assets | | |
Fixed maturity securities, fair value | 3,433.1 | 3,130.4 |
Recurring | U.S. corporate | Level 1 | | |
Financial Assets | | |
Fixed maturity securities, fair value | 0 | 0 |
Recurring | U.S. corporate | Level 2 | | |
Financial Assets | | |
Fixed maturity securities, fair value | 3,378.3 | 3,094.8 |
Recurring | U.S. corporate | Level 3 | | |
Financial Assets | | |
Fixed maturity securities, fair value | 54.8 | 35.6 |
Recurring | Foreign corporate | | |
Financial Assets | | |
Fixed maturity securities, fair value | 1,564 | 1,397.5 |
Recurring | Foreign corporate | Level 1 | | |
Financial Assets | | |
Fixed maturity securities, fair value | 0 | 0 |
Recurring | Foreign corporate | Level 2 | | |
Financial Assets | | |
Fixed maturity securities, fair value | 1,553.7 | 1,390.4 |
Recurring | Foreign corporate | Level 3 | | |
Financial Assets | | |
Fixed maturity securities, fair value | 10.3 | 7.1 |
Recurring | Mutual funds | | |
Financial Assets | | |
Equity securities at fair value | 24.3 | 16.6 |
Recurring | Mutual funds | Level 1 | | |
Financial Assets | | |
Equity securities at fair value | 14.3 | 16.6 |
Recurring | Mutual funds | Level 2 | | |
Financial Assets | | |
Equity securities at fair value | 0 | 0 |
Recurring | Mutual funds | Level 3 | | |
Financial Assets | | |
Equity securities at fair value | 10 | 0 |
Recurring | Common stocks | | |
Financial Assets | | |
Equity securities at fair value | 2.7 | 17.9 |
Recurring | Common stocks | Level 1 | | |
Financial Assets | | |
Equity securities at fair value | 2.7 | 17.2 |
Recurring | Common stocks | Level 2 | | |
Financial Assets | | |
Equity securities at fair value | 0 | 0.7 |
Recurring | Common stocks | Level 3 | | |
Financial Assets | | |
Equity securities at fair value | 0 | 0 |
Recurring | Non-redeemable preferred stocks | | |
Financial Assets | | |
Equity securities at fair value | 192.4 | 188.5 |
Recurring | Non-redeemable preferred stocks | Level 1 | | |
Financial Assets | | |
Equity securities at fair value | 0 | 0 |
Recurring | Non-redeemable preferred stocks | Level 2 | | |
Financial Assets | | |
Equity securities at fair value | 192.4 | 188.5 |
Recurring | Non-redeemable preferred stocks | Level 3 | | |
Financial Assets | | |
Equity securities at fair value | $ 0 | $ 0 |