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Fair Value Measurements - Fair Value of Assets Measured on Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract]
Debt securities, Estimated fair value	$ 7,227.2	$ 8,169.6
Equity securities, estimated fair value	407.3	279.5
Mortgage loans held for sale	13.1	15.9
Total assets	7,647.6	8,465.0
U.S. Treasury bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract]
Debt securities, Estimated fair value	128.1	302.8
Municipal bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract]
Debt securities, Estimated fair value	1,196.1	1,478.7
Foreign government bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract]
Debt securities, Estimated fair value	206.9	193.7
Governmental agency bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract]
Debt securities, Estimated fair value	217.5	228.6
Governmental agency mortgage-backed securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract]
Debt securities, Estimated fair value	4,170.4	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	880.1	921.4
Foreign corporate debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract]
Debt securities, Estimated fair value	428.1	442.0
Common Stock
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract]
Equity securities, estimated fair value	396.0	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	407.3	279.5
Mortgage loans held for sale	0.0	0.0
Total assets	407.3	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	396.0	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	7,227.2	8,169.6
Equity securities, estimated fair value	0.0	0.0
Mortgage loans held for sale	11.9	13.9
Total assets	7,239.1	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	128.1	302.8
Level 2 | Municipal bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract]
Debt securities, Estimated fair value	1,196.1	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	206.9	193.7
Level 2 | Governmental agency bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract]
Debt securities, Estimated fair value	217.5	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	4,170.4	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	880.1	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	428.1	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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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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