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Fair Value Measurements - Fair Value of Assets Measured on Recurring Basis (Details) - USD ($) $ in Millions | Sep. 30, 2024 | Dec. 31, 2023
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Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | $ 7,015.3 | $ 7,157.5
Equity securities, estimated fair value | 404.8 | 436.9
Mortgage loans held for sale | 13.2 | 13.1
Total assets | 7,433.3 | 7,607.5
U.S. Treasury bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 190.4 | 199.3
Municipal bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 731.4 | 1,245.8
Foreign government bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 226.6 | 219.3
Governmental agency bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 222.0 | 195.4
Governmental agency mortgage-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 4,285.9 | 3,875.7
U.S. corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 864.5 | 958.4
Foreign corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 494.5 | 463.6
Common Stock |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Equity securities, estimated fair value | 392.2 | 424.5
Preferred Stock |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Equity securities, estimated fair value | 12.6 | 12.4
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 | 404.8 | 436.9
Mortgage loans held for sale | 0.0 | 0.0
Total assets | 404.8 | 436.9
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 | 392.2 | 424.5
Level 1 | Preferred Stock |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Equity securities, estimated fair value | 12.6 | 12.4
Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 7,015.3 | 7,157.5
Equity securities, estimated fair value | 0.0 | 0.0
Mortgage loans held for sale | 12.7 | 11.8
Total assets | 7,028.0 | 7,169.3
Level 2 | U.S. Treasury bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 190.4 | 199.3
Level 2 | Municipal bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 731.4 | 1,245.8
Level 2 | Foreign government bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 226.6 | 219.3
Level 2 | Governmental agency bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 222.0 | 195.4
Level 2 | Governmental agency mortgage-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 4,285.9 | 3,875.7
Level 2 | U.S. corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 864.5 | 958.4
Level 2 | Foreign corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 494.5 | 463.6
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 | 0.5 | 1.3
Total assets | 0.5 | 1.3
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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