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Fair Value of Financial Instruments - Financial Instruments Measured and Carried at Fair Value (Details) - USD ($)
$ in Millions
Mar. 31, 2025
Dec. 31, 2024
Financial Assets    
Total fixed maturity securities $ 5,457.7 $ 5,387.9
Equity securities 64.3 66.5
Limited partnership interests (Carried under Fair Value Option, 2025 $34900000 and 2024, $28900000) 1,125.6 1,121.3
Mortgage-backed securities    
Financial Assets    
Total fixed maturity securities 812.2 755.8
Other, including U.S. Treasury securities    
Financial Assets    
Total fixed maturity securities 363.3 357.6
Municipal bonds    
Financial Assets    
Total fixed maturity securities 1,155.9 1,150.8
Foreign government bonds    
Financial Assets    
Total fixed maturity securities 13.3 13.1
Corporate bonds    
Financial Assets    
Total fixed maturity securities 1,779.9 1,782.4
Other asset-backed securities    
Financial Assets    
Total fixed maturity securities 1,333.1 1,328.2
Recurring    
Financial Assets    
Limited partnership interests (Carried under Fair Value Option, 2025 $34900000 and 2024, $28900000) 34.9 28.9
Recurring | Carrying Amount    
Financial Assets    
Total fixed maturity securities 5,457.7 5,387.9
Equity securities 64.3 66.5
Limited partnership interests (Carried under Fair Value Option, 2025 $34900000 and 2024, $28900000) 34.9 28.9
Short-term investments 126.0 101.1
Other investments 13.8 18.5
Totals 5,696.7 5,602.9
Separate Account (variable annuity) assets 3,568.3 3,708.8
Financial Liabilities 76.9 79.6
Recurring | Carrying Amount | Mortgage-backed securities    
Financial Assets    
Total fixed maturity securities 812.2 755.8
Recurring | Carrying Amount | Other, including U.S. Treasury securities    
Financial Assets    
Total fixed maturity securities 363.3 357.6
Recurring | Carrying Amount | Municipal bonds    
Financial Assets    
Total fixed maturity securities 1,155.9 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 1,780.0 1,782.4
Recurring | Carrying Amount | Other asset-backed securities    
Financial Assets    
Total fixed maturity securities 1,333.0 1,328.2
Recurring | Fair Value    
Financial Assets    
Total fixed maturity securities 5,457.7 5,387.9
Equity securities 64.3 66.5
Limited partnership interests (Carried under Fair Value Option, 2025 $34900000 and 2024, $28900000) 34.9 28.9
Short-term investments 126.0 101.1
Other investments 13.8 18.5
Totals 5,696.7 5,602.9
Separate Account (variable annuity) assets 3,568.3 3,708.8
Financial Liabilities 76.9 79.6
Recurring | Fair Value | Mortgage-backed securities    
Financial Assets    
Total fixed maturity securities 812.2 755.8
Recurring | Fair Value | Other, including U.S. Treasury securities    
Financial Assets    
Total fixed maturity securities 363.3 357.6
Recurring | Fair Value | Municipal bonds    
Financial Assets    
Total fixed maturity securities 1,155.9 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 1,780.0 1,782.4
Recurring | Fair Value | Other asset-backed securities    
Financial Assets    
Total fixed maturity securities 1,333.0 1,328.2
Recurring | Fair Value | Level 1    
Financial Assets    
Total fixed maturity securities 32.5 32.4
Equity securities 1.3 1.4
Limited partnership interests (Carried under Fair Value Option, 2025 $34900000 and 2024, $28900000) 0.0 0.0
Short-term investments 126.0 101.1
Other investments 0.0 0.0
Totals 159.8 134.9
Separate Account (variable annuity) assets 3,568.3 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 24.8 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 7.7 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 4,941.0 4,855.6
Equity securities 59.0 60.9
Limited partnership interests (Carried under Fair Value Option, 2025 $34900000 and 2024, $28900000) 0.0 0.0
Short-term investments 0.0 0.0
Other investments 13.8 18.5
Totals 5,013.8 4,935.0
Separate Account (variable annuity) assets 0.0 0.0
Financial Liabilities 2.6 4.1
Recurring | Fair Value | Level 2 | Mortgage-backed securities    
Financial Assets    
Total fixed maturity securities 812.2 755.8
Recurring | Fair Value | Level 2 | Other, including U.S. Treasury securities    
Financial Assets    
Total fixed maturity securities 338.5 332.9
Recurring | Fair Value | Level 2 | Municipal bonds    
Financial Assets    
Total fixed maturity securities 1,079.8 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 1,431.3 1,423.4
Recurring | Fair Value | Level 2 | Other asset-backed securities    
Financial Assets    
Total fixed maturity securities 1,265.9 1,254.5
Recurring | Fair Value | Level 3    
Financial Assets    
Total fixed maturity securities 484.1 499.9
Equity securities 4.0 4.2
Limited partnership interests (Carried under Fair Value Option, 2025 $34900000 and 2024, $28900000) 34.9 28.9
Short-term investments 0.0 0.0
Other investments 0.0 0.0
Totals 523.0 533.0
Separate Account (variable annuity) assets 0.0 0.0
Financial Liabilities 74.3 75.5
Recurring | Fair Value | Level 3 | External    
Financial Assets    
Total fixed maturity securities 294.1 300.6
Equity securities 4.0 4.2
Limited partnership interests (Carried under Fair Value Option, 2025 $34900000 and 2024, $28900000) 34.9 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 76.1 74.9
Recurring | Fair Value | Level 3 | Municipal bonds | External    
Financial Assets    
Total fixed maturity securities 76.1 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 340.9 351.3
Recurring | Fair Value | Level 3 | Corporate bonds | External    
Financial Assets    
Total fixed maturity securities 150.9 152.0
Recurring | Fair Value | Level 3 | Other asset-backed securities    
Financial Assets    
Total fixed maturity securities 67.1 73.7
Recurring | Fair Value | Level 3 | Other asset-backed securities | External    
Financial Assets    
Total fixed maturity securities $ 67.1 $ 73.7