Fair Value Measurements - Fair Value of Assets Measured on Recurring Basis (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	$ 7,434.0	$ 7,265.9
Equity securities, estimated fair value	367.4	386.8
Total assets	7,801.4	7,652.7
U.S. Treasury bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	172.9	175.6
Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	796.5	776.5
Foreign government bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	212.7	211.6
Governmental agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	192.5	189.8
Governmental agency mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	4,608.8	4,502.3
U.S. corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	966.5	925.6
Foreign corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	484.1	484.5
Common Stock
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Equity securities, estimated fair value	355.3	374.7
Preferred Stock
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Equity securities, estimated fair value	12.1	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	367.4	386.8
Total assets	367.4	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	355.3	374.7
Level 1 | Preferred Stock
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Equity securities, estimated fair value	12.1	12.1
Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	7,434.0	7,265.9
Equity securities, estimated fair value	0.0	0.0
Total assets	7,434.0	7,265.9
Level 2 | U.S. Treasury bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	172.9	175.6
Level 2 | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	796.5	776.5
Level 2 | Foreign government bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	212.7	211.6
Level 2 | Governmental agency bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	192.5	189.8
Level 2 | Governmental agency mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	4,608.8	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	966.5	925.6
Level 2 | Foreign corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]
Debt securities, estimated fair value	484.1	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