v3.25.2
Fair Value of Financial Instruments - Financial Instruments Measured and Carried at Fair Value (Details) - USD ($)
$ in Millions
Jun. 30, 2025
Dec. 31, 2024
Financial Assets    
Total fixed maturity securities $ 5,453.3 $ 5,387.9
Equity securities 58.3 66.5
Limited partnership interests 1,137.8 1,121.3
Mortgage-backed securities    
Financial Assets    
Total fixed maturity securities 605.5 755.8
Other, including U.S. Treasury securities    
Financial Assets    
Total fixed maturity securities 363.6 357.6
Municipal bonds    
Financial Assets    
Total fixed maturity securities 1,151.9 1,150.8
Foreign government bonds    
Financial Assets    
Total fixed maturity securities 13.3 13.1
Corporate bonds    
Financial Assets    
Total fixed maturity securities 1,770.8 1,782.4
Other asset-backed securities    
Financial Assets    
Total fixed maturity securities 1,548.2 1,328.2
Recurring    
Financial Assets    
Limited partnership interests 36.6 28.9
Recurring | Carrying Amount    
Financial Assets    
Total fixed maturity securities 5,453.3 5,387.9
Equity securities 58.3 66.5
Limited partnership interests 33.7 28.9
Short-term investments 155.6 101.1
Other investments 15.9 18.5
Totals 5,716.8 5,602.9
Separate Account (variable annuity) assets 3,863.3 3,708.8
Financial Liabilities 78.1 79.6
Recurring | Carrying Amount | Mortgage-backed securities    
Financial Assets    
Total fixed maturity securities 605.5 755.8
Recurring | Carrying Amount | Other, including U.S. Treasury securities    
Financial Assets    
Total fixed maturity securities 363.6 357.6
Recurring | Carrying Amount | Municipal bonds    
Financial Assets    
Total fixed maturity securities 1,151.9 1,150.8
Recurring | Carrying Amount | Foreign government bonds    
Financial Assets    
Total fixed maturity securities 13.3 13.1
Recurring | Carrying Amount | Corporate bonds    
Financial Assets    
Total fixed maturity securities 1,770.8 1,782.4
Recurring | Carrying Amount | Other asset-backed securities    
Financial Assets    
Total fixed maturity securities 1,548.2 1,328.2
Recurring | Fair Value    
Financial Assets    
Total fixed maturity securities 5,453.3 5,387.9
Equity securities 58.3 66.5
Limited partnership interests 33.7 28.9
Short-term investments 155.6 101.1
Other investments 15.9 18.5
Totals 5,716.8 5,602.9
Separate Account (variable annuity) assets 3,863.3 3,708.8
Financial Liabilities 78.1 79.6
Recurring | Fair Value | Mortgage-backed securities    
Financial Assets    
Total fixed maturity securities 605.5 755.8
Recurring | Fair Value | Other, including U.S. Treasury securities    
Financial Assets    
Total fixed maturity securities 363.6 357.6
Recurring | Fair Value | Municipal bonds    
Financial Assets    
Total fixed maturity securities 1,151.9 1,150.8
Recurring | Fair Value | Foreign government bonds    
Financial Assets    
Total fixed maturity securities 13.3 13.1
Recurring | Fair Value | Corporate bonds    
Financial Assets    
Total fixed maturity securities 1,770.8 1,782.4
Recurring | Fair Value | Other asset-backed securities    
Financial Assets    
Total fixed maturity securities 1,548.2 1,328.2
Recurring | Fair Value | Level 1    
Financial Assets    
Total fixed maturity securities 29.8 32.4
Equity securities 1.3 1.4
Limited partnership interests 0.0 0.0
Short-term investments 155.6 101.1
Other investments 0.0 0.0
Totals 186.7 134.9
Separate Account (variable annuity) assets 3,863.3 3,708.8
Financial Liabilities 0.0 0.0
Recurring | Fair Value | Level 1 | Mortgage-backed securities    
Financial Assets    
Total fixed maturity securities 0.0 0.0
Recurring | Fair Value | Level 1 | Other, including U.S. Treasury securities    
Financial Assets    
Total fixed maturity securities 24.9 24.7
Recurring | Fair Value | Level 1 | Municipal bonds    
Financial Assets    
Total fixed maturity securities 0.0 0.0
Recurring | Fair Value | Level 1 | Foreign government bonds    
Financial Assets    
Total fixed maturity securities 0.0 0.0
Recurring | Fair Value | Level 1 | Corporate bonds    
Financial Assets    
Total fixed maturity securities 4.9 7.7
Recurring | Fair Value | Level 1 | Other asset-backed securities    
Financial Assets    
Total fixed maturity securities 0.0 0.0
Recurring | Fair Value | Level 2    
Financial Assets    
Total fixed maturity securities 4,942.4 4,855.6
Equity securities 52.7 60.9
Limited partnership interests 0.0 0.0
Short-term investments 0.0 0.0
Other investments 15.9 18.5
Totals 5,011.0 4,935.0
Separate Account (variable annuity) assets 0.0 0.0
Financial Liabilities 4.1 4.1
Recurring | Fair Value | Level 2 | Mortgage-backed securities    
Financial Assets    
Total fixed maturity securities 605.5 755.8
Recurring | Fair Value | Level 2 | Other, including U.S. Treasury securities    
Financial Assets    
Total fixed maturity securities 338.7 332.9
Recurring | Fair Value | Level 2 | Municipal bonds    
Financial Assets    
Total fixed maturity securities 1,076.0 1,075.9
Recurring | Fair Value | Level 2 | Foreign government bonds    
Financial Assets    
Total fixed maturity securities 13.3 13.1
Recurring | Fair Value | Level 2 | Corporate bonds    
Financial Assets    
Total fixed maturity securities 1,423.9 1,423.4
Recurring | Fair Value | Level 2 | Other asset-backed securities    
Financial Assets    
Total fixed maturity securities 1,485.0 1,254.5
Recurring | Fair Value | Level 3    
Financial Assets    
Total fixed maturity securities 481.1 499.9
Equity securities 4.3 4.2
Limited partnership interests 33.7 28.9
Short-term investments 0.0 0.0
Other investments 0.0 0.0
Totals 519.1 533.0
Separate Account (variable annuity) assets 0.0 0.0
Financial Liabilities 74.0 75.5
Recurring | Fair Value | Level 3 | External    
Financial Assets    
Total fixed maturity securities 277.4 300.6
Equity securities 4.3 4.2
Limited partnership interests 33.7 28.9
Financial Liabilities 0.0 0.0
Recurring | Fair Value | Level 3 | Mortgage-backed securities    
Financial Assets    
Total fixed maturity securities 0.0 0.0
Recurring | Fair Value | Level 3 | Mortgage-backed securities | External    
Financial Assets    
Total fixed maturity securities 0.0 0.0
Recurring | Fair Value | Level 3 | Other, including U.S. Treasury securities    
Financial Assets    
Total fixed maturity securities 0.0 0.0
Recurring | Fair Value | Level 3 | Municipal bonds    
Financial Assets    
Total fixed maturity securities 75.9 74.9
Recurring | Fair Value | Level 3 | Municipal bonds | External    
Financial Assets    
Total fixed maturity securities 75.9 74.9
Recurring | Fair Value | Level 3 | Foreign government bonds    
Financial Assets    
Total fixed maturity securities 0.0 0.0
Recurring | Fair Value | Level 3 | Corporate bonds    
Financial Assets    
Total fixed maturity securities 342.0 351.3
Recurring | Fair Value | Level 3 | Corporate bonds | External    
Financial Assets    
Total fixed maturity securities 138.3 152.0
Recurring | Fair Value | Level 3 | Other asset-backed securities    
Financial Assets    
Total fixed maturity securities 63.2 73.7
Recurring | Fair Value | Level 3 | Other asset-backed securities | External    
Financial Assets    
Total fixed maturity securities $ 63.2 $ 73.7