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Fair Value Measurements - Fair Value of Assets Measured on Recurring Basis (Details) - USD ($) $ in Millions | Jun. 30, 2023 | Dec. 31, 2022
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Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | $ 7,454.5 | $ 8,169.6
Equity securities, estimated fair value | 430.9 | 279.5
Mortgage loans held for sale | 13.7 | 15.9
Total assets | 7,899.1 | 8,465.0
U.S. Treasury bonds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 124.4 | 302.8
Municipal bonds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 1,240.0 | 1,478.7
Foreign government bonds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 215.9 | 193.7
Governmental agency bonds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 224.5 | 228.6
Governmental agency mortgage-backed securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 4,311.4 | 4,602.4
U.S. corporate debt securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 897.7 | 921.4
Foreign corporate debt securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 440.6 | 442.0
Common Stock |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Equity securities, estimated fair value | 419.4 | 268.1
Preferred Stock |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Equity securities, estimated fair value | 11.5 | 11.4
Level 1 |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 0.0 | 0.0
Equity securities, estimated fair value | 430.9 | 279.5
Mortgage loans held for sale | 0.0 | 0.0
Total assets | 430.9 | 279.5
Level 1 | U.S. Treasury bonds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 0.0 | 0.0
Level 1 | Municipal bonds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 0.0 | 0.0
Level 1 | Foreign government bonds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 0.0 | 0.0
Level 1 | Governmental agency bonds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring 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 And Nonrecurring 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 And Nonrecurring 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 And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 0.0 | 0.0
Level 1 | Common Stock |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Equity securities, estimated fair value | 419.4 | 268.1
Level 1 | Preferred Stock |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Equity securities, estimated fair value | 11.5 | 11.4
Level 2 |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 7,454.5 | 8,169.6
Equity securities, estimated fair value | 0.0 | 0.0
Mortgage loans held for sale | 12.5 | 13.9
Total assets | 7,467.0 | 8,183.5
Level 2 | U.S. Treasury bonds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 124.4 | 302.8
Level 2 | Municipal bonds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 1,240.0 | 1,478.7
Level 2 | Foreign government bonds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 215.9 | 193.7
Level 2 | Governmental agency bonds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 224.5 | 228.6
Level 2 | Governmental agency mortgage-backed securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 4,311.4 | 4,602.4
Level 2 | U.S. corporate debt securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 897.7 | 921.4
Level 2 | Foreign corporate debt securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 440.6 | 442.0
Level 2 | Common Stock |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Equity securities, estimated fair value | 0.0 | 0.0
Level 2 | Preferred Stock |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Equity securities, estimated fair value | 0.0 | 0.0
Level 3 |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring 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 | 1.2 | 2.0
Total assets | 1.2 | 2.0
Level 3 | U.S. Treasury bonds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 0.0 | 0.0
Level 3 | Municipal bonds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 0.0 | 0.0
Level 3 | Foreign government bonds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 0.0 | 0.0
Level 3 | Governmental agency bonds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring 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 And Nonrecurring 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 And Nonrecurring 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 And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 0.0 | 0.0
Level 3 | Common Stock |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Equity securities, estimated fair value | 0.0 | 0.0
Level 3 | Preferred Stock |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Equity securities, estimated fair value | $ 0.0 | $ 0.0

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