Fair Value Measurements - Fair Value of Assets Measured on Recurring Basis (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
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