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Fair Value Measurements - Fair Value of Assets Measured on Recurring Basis (Details) - USD ($) $ in Millions | Dec. 31, 2023 | Dec. 31, 2022
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Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | $ 7,157.5 | $ 8,169.6
Equity securities, estimated fair value | 436.9 | 279.5
Mortgage loans held for sale | 13.1 | 15.9
Total assets | 7,607.5 | 8,465.0
U.S. Treasury bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 199.3 | 302.8
Municipal bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 1,245.8 | 1,478.7
Foreign government bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 219.3 | 193.7
Governmental agency bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 195.4 | 228.6
Governmental agency mortgage-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 3,875.7 | 4,602.4
U.S. corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 958.4 | 921.4
Foreign corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 463.6 | 442.0
Common Stock |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Equity securities, estimated fair value | 424.5 | 268.1
Preferred Stock |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Equity securities, estimated fair value | 12.4 | 11.4
Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Equity securities, estimated fair value | 436.9 | 279.5
Mortgage loans held for sale | 0.0 | 0.0
Total assets | 436.9 | 279.5
Level 1 | U.S. Treasury bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Level 1 | Municipal bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Level 1 | Foreign government bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Level 1 | Governmental agency bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Level 1 | Governmental agency mortgage-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Level 1 | U.S. corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Level 1 | Foreign corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Level 1 | Common Stock |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Equity securities, estimated fair value | 424.5 | 268.1
Level 1 | Preferred Stock |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Equity securities, estimated fair value | 12.4 | 11.4
Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 7,157.5 | 8,169.6
Equity securities, estimated fair value | 0.0 | 0.0
Mortgage loans held for sale | 11.8 | 13.9
Total assets | 7,169.3 | 8,183.5
Level 2 | U.S. Treasury bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 199.3 | 302.8
Level 2 | Municipal bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 1,245.8 | 1,478.7
Level 2 | Foreign government bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 219.3 | 193.7
Level 2 | Governmental agency bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 195.4 | 228.6
Level 2 | Governmental agency mortgage-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 3,875.7 | 4,602.4
Level 2 | U.S. corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 958.4 | 921.4
Level 2 | Foreign corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 463.6 | 442.0
Level 2 | Common Stock |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Equity securities, estimated fair value | 0.0 | 0.0
Level 2 | Preferred Stock |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Equity securities, estimated fair value | 0.0 | 0.0
Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Equity securities, estimated fair value | 0.0 | 0.0
Mortgage loans held for sale | 1.3 | 2.0
Total assets | 1.3 | 2.0
Level 3 | U.S. Treasury bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Level 3 | Municipal bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Level 3 | Foreign government bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Level 3 | Governmental agency bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Level 3 | Governmental agency mortgage-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Level 3 | U.S. corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Level 3 | Foreign corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Level 3 | Common Stock |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Equity securities, estimated fair value | 0.0 | 0.0
Level 3 | Preferred Stock |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Equity securities, estimated fair value | $ 0.0 | $ 0.0

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