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Fair Value Measurements - Fair Value of Assets Measured on Recurring Basis (Details) - USD ($) $ in Millions | Mar. 31, 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 | $ 7,624.7 | $ 8,169.6
Equity securities, estimated fair value | 315.2 | 279.5
Mortgage loans held for sale | 13.9 | 15.9
Total assets | 7,953.8 | 8,465.0
U.S. Treasury bonds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 221.2 | 302.8
Municipal bonds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 1,249.1 | 1,478.7
Foreign government bonds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 196.1 | 193.7
Governmental agency bonds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 230.5 | 228.6
Governmental agency mortgage-backed securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 4,444.9 | 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 | 838.3 | 921.4
Foreign corporate debt securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 444.6 | 442.0
Common Stock |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Equity securities, estimated fair value | 303.9 | 268.1
Preferred Stock |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Equity securities, estimated fair value | 11.3 | 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 | 315.2 | 279.5
Mortgage loans held for sale | 0.0 | 0.0
Total assets | 315.2 | 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 | 303.9 | 268.1
Level 1 | Preferred Stock |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Equity securities, estimated fair value | 11.3 | 11.4
Level 2 |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 7,624.7 | 8,169.6
Equity securities, estimated fair value | 0.0 | 0.0
Mortgage loans held for sale | 12.6 | 13.9
Total assets | 7,637.3 | 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 | 221.2 | 302.8
Level 2 | Municipal bonds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 1,249.1 | 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 | 196.1 | 193.7
Level 2 | Governmental agency bonds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract] |  |
Debt securities, Estimated fair value | 230.5 | 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 | 4,444.9 | 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 | 838.3 | 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 | 444.6 | 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 | 1.3 | 2.0
Total assets | 1.3 | 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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- Definition Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI https://asc.fasb.org/extlink&oid=126976982&loc=d3e19207-110258 + Details
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