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Fair Value of Financial Instruments - Financial Instruments Measured and Carried at Fair Value (Details) - USD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
Financial Assets    
Total fixed maturity securities $ 5,235.3 $ 5,185.0 [1]
Equity securities 86.2 99.6 [1]
Carrying Value | Recurring    
Financial Assets    
Total fixed maturity securities 5,235.3 5,185.0
Equity securities 86.2 99.6
Short-term investments 132.9 109.4
Other investments 19.0 38.6
Investments, Fair Value Disclosure, Total 5,473.4 5,432.6
Separate account (variable annuity) assets 3,294.1 2,792.3
Financial Liabilities 86.0 92.5
Carrying Value | Mortgage-backed securities | Recurring    
Financial Assets    
Total fixed maturity securities 653.2 570.4
Carrying Value | Other, including U.S. Treasury securities | Recurring    
Financial Assets    
Total fixed maturity securities 388.8 342.6
Carrying Value | Municipal bonds | Recurring    
Financial Assets    
Total fixed maturity securities 1,270.1 1,269.7
Carrying Value | Foreign government bonds | Recurring    
Financial Assets    
Total fixed maturity securities 22.1 33.6
Carrying Value | Corporate bonds | Recurring    
Financial Assets    
Total fixed maturity securities 1,772.7 1,901.7
Carrying Value | Other asset-backed securities | Recurring    
Financial Assets    
Total fixed maturity securities 1,128.4 1,067.0
Fair Value | Recurring    
Financial Assets    
Total fixed maturity securities 5,235.3 5,185.0
Equity securities 86.2 99.6
Short-term investments 132.9 109.4
Other investments 19.0 38.6
Investments, Fair Value Disclosure, Total 5,473.4 5,432.6
Separate account (variable annuity) assets 3,294.1 2,792.3
Financial Liabilities 86.0 92.5
Fair Value | Level 1 | Recurring    
Financial Assets    
Total fixed maturity securities 55.7 36.8
Equity securities 17.9 23.3
Short-term investments 132.9 109.4
Other investments 0.0 0.0
Investments, Fair Value Disclosure, Total 206.5 169.5
Separate account (variable annuity) assets 3,294.1 2,792.3
Financial Liabilities 0.0 0.0
Fair Value | Level 2 | Recurring    
Financial Assets    
Total fixed maturity securities 4,665.6 4,724.9
Equity securities 63.8 74.3
Short-term investments 0.0 0.0
Other investments 19.0 38.6
Investments, Fair Value Disclosure, Total 4,748.4 4,837.8
Separate account (variable annuity) assets 0.0 0.0
Financial Liabilities 3.6 1.2
Fair Value | Level 3 | Recurring    
Financial Assets    
Total fixed maturity securities 514.0 423.3
Equity securities 4.5 2.0
Short-term investments 0.0 0.0
Other investments 0.0 0.0
Investments, Fair Value Disclosure, Total 518.5 425.3
Separate account (variable annuity) assets 0.0 0.0
Financial Liabilities 82.4 91.3
Fair Value | Mortgage-backed securities | Recurring    
Financial Assets    
Total fixed maturity securities 653.2 570.4
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 653.2 567.8
Fair Value | Mortgage-backed securities | Level 3 | Recurring    
Financial Assets    
Total fixed maturity securities 0.0 2.6
Fair Value | Other, including U.S. Treasury securities | Recurring    
Financial Assets    
Total fixed maturity securities 388.8 342.6
Fair Value | Other, including U.S. Treasury securities | Level 1 | Recurring    
Financial Assets    
Total fixed maturity securities 45.6 24.6
Fair Value | Other, including U.S. Treasury securities | Level 2 | Recurring    
Financial Assets    
Total fixed maturity securities 343.2 318.0
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 1,270.1 1,269.7
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 1,196.1 1,215.3
Fair Value | Municipal bonds | Level 3 | Recurring    
Financial Assets    
Total fixed maturity securities 74.0 54.4
Fair Value | Foreign government bonds | Recurring    
Financial Assets    
Total fixed maturity securities 22.1 33.6
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 22.1 33.6
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 1,772.7 1,901.7
Fair Value | Corporate bonds | Level 1 | Recurring    
Financial Assets    
Total fixed maturity securities 10.1 12.2
Fair Value | Corporate bonds | Level 2 | Recurring    
Financial Assets    
Total fixed maturity securities 1,420.1 1,628.2
Fair Value | Corporate bonds | Level 3 | Recurring    
Financial Assets    
Total fixed maturity securities 342.5 261.3
Fair Value | Other asset-backed securities | Recurring    
Financial Assets    
Total fixed maturity securities 1,128.4 1,067.0
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 1,030.9 962.0
Fair Value | Other asset-backed securities | Level 3 | Recurring    
Financial Assets    
Total fixed maturity securities $ 97.5 $ 105.0
[1] Recast for the adoption of ASU 2018-12. See Note 1 of the Consolidated Financial Statements