v3.26.3
Fair Value of Financial Instruments - Financial Instruments Measured and Carried at Fair Value (Details) - USD ($)
$ in Millions
Sep. 30, 2026
Dec. 31, 2025
Financial Assets    
Total fixed maturity securities $ 5,520.8 $ 5,714.6
Equity securities 37.5 41.9
Fair Value Measured at Net Asset Value Per Share    
Financial Assets    
Limited partnership investment 44.0  
Carrying Amount    
Financial Assets    
Total fixed maturity securities 5,520.8 5,714.6
Equity securities 37.5 41.9
Short-term investments 117.1 210.5
Other investments 18.7 21.6
Totals 5,694.1 5,988.6
Separate account (variable annuity) assets 4,505.6 4,157.4
Financial liabilities 73.5 78.9
Carrying Amount | Mortgage-backed securities    
Financial Assets    
Total fixed maturity securities 652.1 689.0
Carrying Amount | Other, including U.S. Treasury securities    
Financial Assets    
Total fixed maturity securities 297.9 325.8
Carrying Amount | Municipal bonds    
Financial Assets    
Total fixed maturity securities 1,079.9 1,177.1
Carrying Amount | Foreign government bonds    
Financial Assets    
Total fixed maturity securities 7.6 10.0
Carrying Amount | Corporate bonds    
Financial Assets    
Total fixed maturity securities 1,835.2 1,910.3
Carrying Amount | Other asset-backed securities    
Financial Assets    
Total fixed maturity securities 1,648.1 1,602.4
Fair Value    
Financial Assets    
Total fixed maturity securities 5,520.8 5,714.6
Equity securities 37.5 41.9
Short-term investments 117.1 210.5
Other investments 18.7 21.6
Totals 5,694.1 5,988.6
Separate account (variable annuity) assets 4,505.6 4,157.4
Financial liabilities 73.5 78.9
Fair Value | Mortgage-backed securities    
Financial Assets    
Total fixed maturity securities 652.1 689.0
Fair Value | Other, including U.S. Treasury securities    
Financial Assets    
Total fixed maturity securities 297.9 325.8
Fair Value | Municipal bonds    
Financial Assets    
Total fixed maturity securities 1,079.9 1,177.1
Fair Value | Foreign government bonds    
Financial Assets    
Total fixed maturity securities 7.6 10.0
Fair Value | Corporate bonds    
Financial Assets    
Total fixed maturity securities 1,835.2 1,910.3
Fair Value | Other asset-backed securities    
Financial Assets    
Total fixed maturity securities 1,648.1 1,602.4
Fair Value | Level 1    
Financial Assets    
Total fixed maturity securities 46.3 45.7
Equity securities 1.2 1.3
Short-term investments 117.1 210.5
Other investments 0.0 0.0
Totals 164.6 257.5
Separate account (variable annuity) assets 4,505.6 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 45.8 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 5,050.1 5,203.6
Equity securities 32.7 36.8
Short-term investments 0.0 0.0
Other investments 18.7 21.6
Totals 5,101.5 5,262.0
Separate account (variable annuity) assets 0.0 0.0
Financial liabilities 7.1 5.8
Fair Value | Level 2 | Mortgage-backed securities    
Financial Assets    
Total fixed maturity securities 652.1 689.0
Fair Value | Level 2 | Other, including U.S. Treasury securities    
Financial Assets    
Total fixed maturity securities 252.1 285.0
Fair Value | Level 2 | Municipal bonds    
Financial Assets    
Total fixed maturity securities 999.4 1,098.7
Fair Value | Level 2 | Foreign government bonds    
Financial Assets    
Total fixed maturity securities 7.6 10.0
Fair Value | Level 2 | Corporate bonds    
Financial Assets    
Total fixed maturity securities 1,521.2 1,562.8
Fair Value | Level 2 | Other asset-backed securities    
Financial Assets    
Total fixed maturity securities 1,617.7 1,558.1
Fair Value | Level 3    
Financial Assets    
Total fixed maturity securities 424.4 465.3
Equity securities 3.6 3.8
Short-term investments 0.0 0.0
Other investments 0.0 0.0
Totals 428.0 469.1
Separate account (variable annuity) assets 0.0 0.0
Financial liabilities 66.4 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 80.5 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 313.5 342.6
Fair Value | Level 3 | Other asset-backed securities    
Financial Assets    
Total fixed maturity securities $ 30.4 $ 44.3