Fair Value Measurements - Fair Value of Assets Measured on Recurring Basis (Details) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	$ 8,018.9	$ 7,265.9
Equity securities, estimated fair value	396.0	386.8
Total assets	8,414.9	7,652.7
U.S. Treasury bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	170.2	175.6
Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	876.7	776.5
Foreign government bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	225.5	211.6
Governmental agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	206.3	189.8
Governmental agency mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	5,037.9	4,502.3
U.S. corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	1,010.0	925.6
Foreign corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	492.3	484.5
Common Stock
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Equity securities, estimated fair value	384.2	374.7
Preferred Stock
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Equity securities, estimated fair value	11.8	12.1
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	396.0	386.8
Total assets	396.0	386.8
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	384.2	374.7
Level 1 | Preferred Stock
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Equity securities, estimated fair value	11.8	12.1
Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	8,018.9	7,265.9
Equity securities, estimated fair value	0.0	0.0
Total assets	8,018.9	7,265.9
Level 2 | U.S. Treasury bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	170.2	175.6
Level 2 | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	876.7	776.5
Level 2 | Foreign government bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	225.5	211.6
Level 2 | Governmental agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	206.3	189.8
Level 2 | Governmental agency mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	5,037.9	4,502.3
Level 2 | U.S. corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	1,010.0	925.6
Level 2 | Foreign corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	492.3	484.5
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
Total assets	0.0	0.0
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