v3.26.1
Fair Value of Financial Instruments - Financial Instruments Measured and Carried at Fair Value (Details) - USD ($)
$ in Millions
Jun. 30, 2026
Dec. 31, 2025
Financial Assets    
Total fixed maturity securities $ 5,767.0 $ 5,714.6
Equity securities 40.3 41.9
Fair Value Measured at Net Asset Value Per Share    
Financial Assets    
Limited partnership investment 26.9  
Carrying Amount    
Financial Assets    
Total fixed maturity securities 5,767.0 5,714.6
Equity securities 40.3 41.9
Short-term investments 184.8 210.5
Other investments 24.4 21.6
Totals 6,016.5 5,988.6
Separate account (variable annuity) assets 4,469.8 4,157.4
Financial Liabilities 79.9 78.9
Carrying Amount | Mortgage-backed securities    
Financial Assets    
Total fixed maturity securities 662.2 689.0
Carrying Amount | Other, including U.S. Treasury securities    
Financial Assets    
Total fixed maturity securities 313.2 325.8
Carrying Amount | Municipal bonds    
Financial Assets    
Total fixed maturity securities 1,163.5 1,177.1
Carrying Amount | Foreign government bonds    
Financial Assets    
Total fixed maturity securities 7.9 10.0
Carrying Amount | Corporate bonds    
Financial Assets    
Total fixed maturity securities 1,954.5 1,910.3
Carrying Amount | Other asset-backed securities    
Financial Assets    
Total fixed maturity securities 1,665.7 1,602.4
Fair Value    
Financial Assets    
Total fixed maturity securities 5,767.0 5,714.6
Equity securities 40.3 41.9
Short-term investments 184.8 210.5
Other investments 24.4 21.6
Totals 6,016.5 5,988.6
Separate account (variable annuity) assets 4,469.8 4,157.4
Financial Liabilities 79.9 78.9
Fair Value | Mortgage-backed securities    
Financial Assets    
Total fixed maturity securities 662.2 689.0
Fair Value | Other, including U.S. Treasury securities    
Financial Assets    
Total fixed maturity securities 313.2 325.8
Fair Value | Municipal bonds    
Financial Assets    
Total fixed maturity securities 1,163.5 1,177.1
Fair Value | Foreign government bonds    
Financial Assets    
Total fixed maturity securities 7.9 10.0
Fair Value | Corporate bonds    
Financial Assets    
Total fixed maturity securities 1,954.5 1,910.3
Fair Value | Other asset-backed securities    
Financial Assets    
Total fixed maturity securities 1,665.7 1,602.4
Fair Value | Level 1    
Financial Assets    
Total fixed maturity securities 38.8 45.7
Equity securities 1.3 1.3
Short-term investments 184.8 210.5
Other investments 0.0 0.0
Totals 224.9 257.5
Separate account (variable annuity) assets 4,469.8 4,157.4
Financial Liabilities 0.0 0.0
Fair Value | Level 1 | Mortgage-backed securities    
Financial Assets    
Total fixed maturity securities 0.0 0.0
Fair Value | Level 1 | Other, including U.S. Treasury securities    
Financial Assets    
Total fixed maturity securities 38.3 40.8
Fair Value | Level 1 | Municipal bonds    
Financial Assets    
Total fixed maturity securities 0.0 0.0
Fair Value | Level 1 | Foreign government bonds    
Financial Assets    
Total fixed maturity securities 0.0 0.0
Fair Value | Level 1 | Corporate bonds    
Financial Assets    
Total fixed maturity securities 0.5 4.9
Fair Value | Level 1 | Other asset-backed securities    
Financial Assets    
Total fixed maturity securities 0.0 0.0
Fair Value | Level 2    
Financial Assets    
Total fixed maturity securities 5,287.5 5,203.6
Equity securities 35.5 36.8
Short-term investments 0.0 0.0
Other investments 24.4 21.6
Totals 5,347.4 5,262.0
Separate account (variable annuity) assets 0.0 0.0
Financial Liabilities 7.6 5.8
Fair Value | Level 2 | Mortgage-backed securities    
Financial Assets    
Total fixed maturity securities 662.2 689.0
Fair Value | Level 2 | Other, including U.S. Treasury securities    
Financial Assets    
Total fixed maturity securities 274.8 285.0
Fair Value | Level 2 | Municipal bonds    
Financial Assets    
Total fixed maturity securities 1,079.1 1,098.7
Fair Value | Level 2 | Foreign government bonds    
Financial Assets    
Total fixed maturity securities 7.9 10.0
Fair Value | Level 2 | Corporate bonds    
Financial Assets    
Total fixed maturity securities 1,632.1 1,562.8
Fair Value | Level 2 | Other asset-backed securities    
Financial Assets    
Total fixed maturity securities 1,631.4 1,558.1
Fair Value | Level 3    
Financial Assets    
Total fixed maturity securities 440.6 465.3
Equity securities 3.6 3.8
Short-term investments 0.0 0.0
Other investments 0.0 0.0
Totals 444.2 469.1
Separate account (variable annuity) assets 0.0 0.0
Financial Liabilities 72.3 73.1
Fair Value | Level 3 | Mortgage-backed securities    
Financial Assets    
Total fixed maturity securities 0.0 0.0
Fair Value | Level 3 | Other, including U.S. Treasury securities    
Financial Assets    
Total fixed maturity securities 0.0 0.0
Fair Value | Level 3 | Municipal bonds    
Financial Assets    
Total fixed maturity securities 84.4 78.4
Fair Value | Level 3 | Foreign government bonds    
Financial Assets    
Total fixed maturity securities 0.0 0.0
Fair Value | Level 3 | Corporate bonds    
Financial Assets    
Total fixed maturity securities 321.9 342.6
Fair Value | Level 3 | Other asset-backed securities    
Financial Assets    
Total fixed maturity securities $ 34.3 $ 44.3