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Fair Value of Financial Instruments - Financial Instruments Measured and Carried at Fair Value (Details) - USD ($)
$ in Millions
Dec. 31, 2024
Dec. 31, 2023
Financial Assets    
Total fixed maturity securities $ 5,387.9 $ 5,235.3
Equity securities 66.5 86.2
Limited partnership interests (Carried under Fair Value Option, 2024 $28.9 and 2023, $29.5) 1,121.3 1,138.8
Recurring    
Financial Assets    
Limited partnership interests (Carried under Fair Value Option, 2024 $28.9 and 2023, $29.5) 28.9 29.5
Carrying Amount | Recurring    
Financial Assets    
Total fixed maturity securities 5,387.9 5,235.3
Equity securities 66.5 86.2
Limited partnership interests (Carried under Fair Value Option, 2024 $28.9 and 2023, $29.5) 28.9  
Short-term investments 101.1 132.9
Other investments 18.5 19.0
Other investments 5,602.9 5,473.4
Separate account (variable annuity) assets 3,708.8 3,294.1
Financial Liabilities 79.6 86.0
Carrying Amount | Mortgage-backed securities | Recurring    
Financial Assets    
Total fixed maturity securities 755.8 653.2
Carrying Amount | Other, including U.S. Treasury securities | Recurring    
Financial Assets    
Total fixed maturity securities 357.6 388.8
Carrying Amount | Municipal Bonds | Recurring    
Financial Assets    
Total fixed maturity securities 1,150.8 1,270.1
Carrying Amount | Foreign government bonds | Recurring    
Financial Assets    
Total fixed maturity securities 13.1 22.1
Carrying Amount | Corporate  Bonds | Recurring    
Financial Assets    
Total fixed maturity securities 1,782.4 1,772.7
Carrying Amount | Other asset-backed securities | Recurring    
Financial Assets    
Total fixed maturity securities 1,328.2 1,128.4
Fair Value | Recurring    
Financial Assets    
Total fixed maturity securities 5,387.9 5,235.3
Equity securities 66.5 86.2
Limited partnership interests (Carried under Fair Value Option, 2024 $28.9 and 2023, $29.5) 28.9  
Short-term investments 101.1 132.9
Other investments 18.5 19.0
Other investments 5,602.9 5,473.4
Separate account (variable annuity) assets 3,708.8 3,294.1
Financial Liabilities 79.6 86.0
Fair Value | Level 1 | Recurring    
Financial Assets    
Total fixed maturity securities 32.4 55.7
Equity securities 1.4 17.9
Limited partnership interests (Carried under Fair Value Option, 2024 $28.9 and 2023, $29.5) 0.0  
Short-term investments 101.1 132.9
Other investments 0.0 0.0
Other investments 134.9 206.5
Separate account (variable annuity) assets 3,708.8 3,294.1
Financial Liabilities 0.0 0.0
Fair Value | Level 2 | Recurring    
Financial Assets    
Total fixed maturity securities 4,855.6 4,665.6
Equity securities 60.9 63.8
Limited partnership interests (Carried under Fair Value Option, 2024 $28.9 and 2023, $29.5) 0.0  
Short-term investments 0.0 0.0
Other investments 18.5 19.0
Other investments 4,935.0 4,748.4
Separate account (variable annuity) assets 0.0 0.0
Financial Liabilities 4.1 3.6
Fair Value | Level 3 | Recurring    
Financial Assets    
Total fixed maturity securities 499.9 514.0
Equity securities 4.2 4.5
Limited partnership interests (Carried under Fair Value Option, 2024 $28.9 and 2023, $29.5) 28.9  
Short-term investments 0.0 0.0
Other investments 0.0 0.0
Other investments 533.0 518.5
Separate account (variable annuity) assets 0.0 0.0
Financial Liabilities 75.5 82.4
Fair Value | Mortgage-backed securities | Recurring    
Financial Assets    
Total fixed maturity securities 755.8 653.2
Fair Value | Mortgage-backed securities | Level 1 | Recurring    
Financial Assets    
Total fixed maturity securities 0.0 0.0
Fair Value | Mortgage-backed securities | Level 2 | Recurring    
Financial Assets    
Total fixed maturity securities 755.8 653.2
Fair Value | Mortgage-backed securities | Level 3 | Recurring    
Financial Assets    
Total fixed maturity securities 0.0 0.0
Fair Value | Other, including U.S. Treasury securities | Recurring    
Financial Assets    
Total fixed maturity securities 357.6 388.8
Fair Value | Other, including U.S. Treasury securities | Level 1 | Recurring    
Financial Assets    
Total fixed maturity securities 24.7 45.6
Fair Value | Other, including U.S. Treasury securities | Level 2 | Recurring    
Financial Assets    
Total fixed maturity securities 332.9 343.2
Fair Value | Other, including U.S. Treasury securities | Level 3 | Recurring    
Financial Assets    
Total fixed maturity securities 0.0 0.0
Fair Value | Municipal Bonds | Recurring    
Financial Assets    
Total fixed maturity securities 1,150.8 1,270.1
Fair Value | Municipal Bonds | Level 1 | Recurring    
Financial Assets    
Total fixed maturity securities 0.0 0.0
Fair Value | Municipal Bonds | Level 2 | Recurring    
Financial Assets    
Total fixed maturity securities 1,075.9 1,196.1
Fair Value | Municipal Bonds | Level 3 | Recurring    
Financial Assets    
Total fixed maturity securities 74.9 74.0
Fair Value | Foreign government bonds | Recurring    
Financial Assets    
Total fixed maturity securities 13.1 22.1
Fair Value | Foreign government bonds | Level 1 | Recurring    
Financial Assets    
Total fixed maturity securities 0.0 0.0
Fair Value | Foreign government bonds | Level 2 | Recurring    
Financial Assets    
Total fixed maturity securities 13.1 22.1
Fair Value | Foreign government bonds | Level 3 | Recurring    
Financial Assets    
Total fixed maturity securities 0.0 0.0
Fair Value | Corporate  Bonds | Recurring    
Financial Assets    
Total fixed maturity securities 1,782.4 1,772.7
Fair Value | Corporate  Bonds | Level 1 | Recurring    
Financial Assets    
Total fixed maturity securities 7.7 10.1
Fair Value | Corporate  Bonds | Level 2 | Recurring    
Financial Assets    
Total fixed maturity securities 1,423.4 1,420.1
Fair Value | Corporate  Bonds | Level 3 | Recurring    
Financial Assets    
Total fixed maturity securities 351.3 342.5
Fair Value | Other asset-backed securities | Recurring    
Financial Assets    
Total fixed maturity securities 1,328.2 1,128.4
Fair Value | Other asset-backed securities | Level 1 | Recurring    
Financial Assets    
Total fixed maturity securities 0.0 0.0
Fair Value | Other asset-backed securities | Level 2 | Recurring    
Financial Assets    
Total fixed maturity securities 1,254.5 1,030.9
Fair Value | Other asset-backed securities | Level 3 | Recurring    
Financial Assets    
Total fixed maturity securities $ 73.7 $ 97.5