v3.26.3
Fair Value Measurements - Fair Value of Assets Measured on Recurring Basis (Details) - USD ($)
$ in Millions
Sep. 30, 2026
Dec. 31, 2025
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]    
Debt securities, estimated fair value $ 9,762.6 $ 8,466.7
Equity securities, estimated fair value 505.4 477.6
Total assets 10,268.0 8,944.3
U.S. Treasury bonds    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]    
Debt securities, estimated fair value 247.5 243.6
Municipal bonds    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]    
Debt securities, estimated fair value 1,029.2 960.1
Foreign government bonds    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]    
Debt securities, estimated fair value 236.3 235.2
Governmental agency bonds    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]    
Debt securities, estimated fair value 444.0 261.0
Governmental agency mortgage-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]    
Debt securities, estimated fair value 6,094.2 5,260.8
U.S. corporate debt securities    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]    
Debt securities, estimated fair value 1,234.3 1,034.8
Foreign corporate debt securities    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]    
Debt securities, estimated fair value 477.1 471.2
Common Stock    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]    
Equity securities, estimated fair value 495.6 467.7
Preferred Stock    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]    
Equity securities, estimated fair value 9.8 9.9
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 505.4 477.6
Total assets 505.4 477.6
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 495.6 467.7
Level 1 | Preferred Stock    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]    
Equity securities, estimated fair value 9.8 9.9
Level 2    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]    
Debt securities, estimated fair value 9,762.6 8,466.7
Equity securities, estimated fair value 0.0 0.0
Total assets 9,762.6 8,466.7
Level 2 | U.S. Treasury bonds    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]    
Debt securities, estimated fair value 247.5 243.6
Level 2 | Municipal bonds    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]    
Debt securities, estimated fair value 1,029.2 960.1
Level 2 | Foreign government bonds    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]    
Debt securities, estimated fair value 236.3 235.2
Level 2 | Governmental agency bonds    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]    
Debt securities, estimated fair value 444.0 261.0
Level 2 | Governmental agency mortgage-backed securities    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]    
Debt securities, estimated fair value 6,094.2 5,260.8
Level 2 | U.S. corporate debt securities    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]    
Debt securities, estimated fair value 1,234.3 1,034.8
Level 2 | Foreign corporate debt securities    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract]    
Debt securities, estimated fair value 477.1 471.2
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