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Fair Value - Composition of Investment Portfolio by Major Security Type (Detail) - USD ($)
$ in Millions
Mar. 31, 2024
Dec. 31, 2023
Mar. 31, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities $ 64,956.4 $ 62,168.1 $ 52,813.3
Equity securities 4,081.6 3,830.5 3,873.1
Investments, fair value 69,038.0 65,998.6 56,686.4
Fair value 6,297.8 6,431.3 5,881.0
Nonredeemable preferred stocks      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 886.7 902.1 1,078.8
Nonredeemable preferred stocks | Financials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 834.5 851.9 991.9
Nonredeemable preferred stocks | Utilities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 37.4 35.4 70.5
Nonredeemable preferred stocks | Industrials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 14.8 14.8 16.4
Common equities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 3,194.9 2,928.4 2,794.3
Common equities | Common Stocks, excluding Other Risk      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 3,170.6 2,907.8 2,774.0
Common equities | Other risk investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 24.3 20.6 20.3
Total fixed maturities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 63,629.7 60,378.2 50,289.2
Total fixed maturities | U.S. government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 38,562.7 36,869.4 27,350.1
Total fixed maturities | State and local government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 2,059.8 2,202.8 2,061.6
Total fixed maturities | Foreign government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 15.9 16.3 15.8
Total fixed maturities | Corporate debt securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 12,521.3 11,183.7 10,681.3
Total fixed maturities | Residential mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 383.1 417.2 630.0
Total fixed maturities | Commercial mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 3,851.8 3,939.7 4,503.0
Total fixed maturities | Other asset-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 6,061.2 5,575.4 4,865.8
Total fixed maturities | Debt Securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 53,159.7 50,272.2 40,108.8
Total fixed maturities | Debt Securities | U.S. government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 38,562.7 36,869.4 27,350.1
Total fixed maturities | Debt Securities | State and local government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 2,059.8 2,202.8 2,061.6
Total fixed maturities | Debt Securities | Foreign government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 15.9 16.3 15.8
Total fixed maturities | Debt Securities | Corporate debt securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 12,521.3 11,183.7 10,681.3
Total fixed maturities | Asset-backed Securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 10,296.1 9,932.3 9,998.8
Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 383.1 417.2 630.0
Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 3,851.8 3,939.7 4,503.0
Total fixed maturities | Asset-backed Securities | Other asset-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 6,061.2 5,575.4 4,865.8
Total fixed maturities | Redeemable preferred stocks      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 173.9 173.7 181.6
Total fixed maturities | Redeemable preferred stocks | Financials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 23.2 23.1 40.0
Total fixed maturities | Redeemable preferred stocks | Utilities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 9.6 9.4 8.8
Total fixed maturities | Redeemable preferred stocks | Industrials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 141.1 141.2 132.8
Short-term investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 1,326.7 1,789.9 2,524.1
Fair Value, Inputs, Level 1      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 39,889.4 38,626.4 29,883.8
Equity securities 3,148.1 2,885.3 2,795.7
Investments, fair value 43,037.5 41,511.7 32,679.5
Fair value 0.0 0.0 0.0
Fair Value, Inputs, Level 1 | Nonredeemable preferred stocks      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 0.0 0.0 40.0
Fair Value, Inputs, Level 1 | Nonredeemable preferred stocks | Financials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 0.0 0.0 40.0
Fair Value, Inputs, Level 1 | Nonredeemable preferred stocks | Utilities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 0.0 0.0 0.0
Fair Value, Inputs, Level 1 | Nonredeemable preferred stocks | Industrials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 0.0 0.0 0.0
Fair Value, Inputs, Level 1 | Common equities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 3,148.1 2,885.3 2,755.7
Fair Value, Inputs, Level 1 | Common equities | Common Stocks, excluding Other Risk      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 3,148.1 2,885.3 2,755.7
Fair Value, Inputs, Level 1 | Common equities | Other risk investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 0.0 0.0 0.0
Fair Value, Inputs, Level 1 | Total fixed maturities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 38,562.7 36,869.4 27,359.7
Fair Value, Inputs, Level 1 | Total fixed maturities | Debt Securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 38,562.7 36,869.4 27,350.1
Fair Value, Inputs, Level 1 | Total fixed maturities | Debt Securities | U.S. government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 38,562.7 36,869.4 27,350.1
Fair Value, Inputs, Level 1 | Total fixed maturities | Debt Securities | State and local government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0.0 0.0 0.0
Fair Value, Inputs, Level 1 | Total fixed maturities | Debt Securities | Foreign government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0.0 0.0 0.0
Fair Value, Inputs, Level 1 | Total fixed maturities | Debt Securities | Corporate debt securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0.0 0.0 0.0
Fair Value, Inputs, Level 1 | Total fixed maturities | Asset-backed Securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0.0 0.0 0.0
Fair Value, Inputs, Level 1 | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0.0 0.0 0.0
Fair Value, Inputs, Level 1 | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0.0 0.0 0.0
Fair Value, Inputs, Level 1 | Total fixed maturities | Asset-backed Securities | Other asset-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0.0 0.0 0.0
Fair Value, Inputs, Level 1 | Total fixed maturities | Redeemable preferred stocks      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0.0 0.0 9.6
Fair Value, Inputs, Level 1 | Total fixed maturities | Redeemable preferred stocks | Financials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0.0 0.0 0.0
Fair Value, Inputs, Level 1 | Total fixed maturities | Redeemable preferred stocks | Utilities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0.0 0.0 0.0
Fair Value, Inputs, Level 1 | Total fixed maturities | Redeemable preferred stocks | Industrials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0.0 0.0 9.6
Fair Value, Inputs, Level 1 | Short-term investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 1,326.7 1,757.0 2,524.1
Fair Value, Inputs, Level 2      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 25,064.0 23,538.7 22,929.5
Equity securities 822.7 838.1 955.0
Investments, fair value 25,886.7 24,376.8 23,884.5
Fair value 6,297.8 6,431.3 5,881.0
Fair Value, Inputs, Level 2 | Nonredeemable preferred stocks      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 822.7 838.1 955.0
Fair Value, Inputs, Level 2 | Nonredeemable preferred stocks | Financials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 785.3 802.7 884.5
Fair Value, Inputs, Level 2 | Nonredeemable preferred stocks | Utilities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 37.4 35.4 70.5
Fair Value, Inputs, Level 2 | Nonredeemable preferred stocks | Industrials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 0.0 0.0 0.0
Fair Value, Inputs, Level 2 | Common equities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 0.0 0.0 0.0
Fair Value, Inputs, Level 2 | Common equities | Common Stocks, excluding Other Risk      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 0.0 0.0 0.0
Fair Value, Inputs, Level 2 | Common equities | Other risk investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 0.0 0.0 0.0
Fair Value, Inputs, Level 2 | Total fixed maturities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 25,064.0 23,505.8 22,929.5
Fair Value, Inputs, Level 2 | Total fixed maturities | Debt Securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 14,594.0 13,399.8 12,758.7
Fair Value, Inputs, Level 2 | Total fixed maturities | Debt Securities | U.S. government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0.0 0.0 0.0
Fair Value, Inputs, Level 2 | Total fixed maturities | Debt Securities | State and local government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 2,059.8 2,202.8 2,061.6
Fair Value, Inputs, Level 2 | Total fixed maturities | Debt Securities | Foreign government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 15.9 16.3 15.8
Fair Value, Inputs, Level 2 | Total fixed maturities | Debt Securities | Corporate debt securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 12,518.3 11,180.7 10,681.3
Fair Value, Inputs, Level 2 | Total fixed maturities | Asset-backed Securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 10,296.1 9,932.3 9,998.8
Fair Value, Inputs, Level 2 | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 383.1 417.2 630.0
Fair Value, Inputs, Level 2 | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 3,851.8 3,939.7 4,503.0
Fair Value, Inputs, Level 2 | Total fixed maturities | Asset-backed Securities | Other asset-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 6,061.2 5,575.4 4,865.8
Fair Value, Inputs, Level 2 | Total fixed maturities | Redeemable preferred stocks      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 173.9 173.7 172.0
Fair Value, Inputs, Level 2 | Total fixed maturities | Redeemable preferred stocks | Financials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 23.2 23.1 40.0
Fair Value, Inputs, Level 2 | Total fixed maturities | Redeemable preferred stocks | Utilities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 9.6 9.4 8.8
Fair Value, Inputs, Level 2 | Total fixed maturities | Redeemable preferred stocks | Industrials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 141.1 141.2 123.2
Fair Value, Inputs, Level 2 | Short-term investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0.0 32.9 0.0
Fair Value, Inputs, Level 3      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 3.0 3.0 0.0
Equity securities 110.8 107.1 122.4
Investments, fair value 113.8 110.1 122.4
Fair value 0.0 0.0 0.0
Fair Value, Inputs, Level 3 | Nonredeemable preferred stocks      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 64.0 64.0 83.8
Fair Value, Inputs, Level 3 | Nonredeemable preferred stocks | Financials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 49.2 49.2 67.4
Fair Value, Inputs, Level 3 | Nonredeemable preferred stocks | Utilities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 0.0 0.0 0.0
Fair Value, Inputs, Level 3 | Nonredeemable preferred stocks | Industrials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 14.8 14.8 16.4
Fair Value, Inputs, Level 3 | Common equities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 46.8 43.1 38.6
Fair Value, Inputs, Level 3 | Common equities | Common Stocks, excluding Other Risk      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 22.5 22.5 18.3
Fair Value, Inputs, Level 3 | Common equities | Other risk investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 24.3 20.6 20.3
Fair Value, Inputs, Level 3 | Total fixed maturities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 3.0 3.0 0.0
Fair Value, Inputs, Level 3 | Total fixed maturities | Debt Securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 3.0 3.0 0.0
Fair Value, Inputs, Level 3 | Total fixed maturities | Debt Securities | U.S. government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0.0 0.0 0.0
Fair Value, Inputs, Level 3 | Total fixed maturities | Debt Securities | State and local government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0.0 0.0 0.0
Fair Value, Inputs, Level 3 | Total fixed maturities | Debt Securities | Foreign government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0.0 0.0 0.0
Fair Value, Inputs, Level 3 | Total fixed maturities | Debt Securities | Corporate debt securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 3.0 3.0 0.0
Fair Value, Inputs, Level 3 | Total fixed maturities | Asset-backed Securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0.0 0.0 0.0
Fair Value, Inputs, Level 3 | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0.0 0.0 0.0
Fair Value, Inputs, Level 3 | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0.0 0.0 0.0
Fair Value, Inputs, Level 3 | Total fixed maturities | Asset-backed Securities | Other asset-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0.0 0.0 0.0
Fair Value, Inputs, Level 3 | Total fixed maturities | Redeemable preferred stocks      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0.0 0.0 0.0
Fair Value, Inputs, Level 3 | Total fixed maturities | Redeemable preferred stocks | Financials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0.0 0.0 0.0
Fair Value, Inputs, Level 3 | Total fixed maturities | Redeemable preferred stocks | Utilities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0.0 0.0 0.0
Fair Value, Inputs, Level 3 | Total fixed maturities | Redeemable preferred stocks | Industrials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0.0 0.0 0.0
Fair Value, Inputs, Level 3 | Short-term investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 0.0 0.0 0.0
Carrying (Reported) Amount, Fair Value Disclosure      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 67,276.0 64,231.8 55,648.0
Equity securities 1,639.3 1,683.1 1,938.2
Investments, fair value 68,915.3 65,914.9 57,586.2
Long-term debt 6,889.7 6,888.6 6,389.3
Carrying (Reported) Amount, Fair Value Disclosure | Nonredeemable preferred stocks      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 931.1 977.1 1,197.7
Carrying (Reported) Amount, Fair Value Disclosure | Nonredeemable preferred stocks | Financials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 876.1 922.1 1,102.7
Carrying (Reported) Amount, Fair Value Disclosure | Nonredeemable preferred stocks | Utilities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 40.0 40.0 80.0
Carrying (Reported) Amount, Fair Value Disclosure | Nonredeemable preferred stocks | Industrials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 15.0 15.0 15.0
Carrying (Reported) Amount, Fair Value Disclosure | Common equities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 708.2 706.0 740.5
Carrying (Reported) Amount, Fair Value Disclosure | Common equities | Common Stocks, excluding Other Risk      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 683.9 685.4 720.2
Carrying (Reported) Amount, Fair Value Disclosure | Common equities | Other risk investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 24.3 20.6 20.3
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 65,949.3 62,441.9 53,123.9
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Debt Securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 54,863.3 51,624.9 41,926.0
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Debt Securities | U.S. government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 39,823.8 37,823.2 28,490.9
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Debt Securities | State and local government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 2,200.7 2,338.4 2,222.4
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Debt Securities | Foreign government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 16.9 17.3 16.9
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Debt Securities | Corporate debt securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 12,821.9 11,446.0 11,195.8
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Asset-backed Securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 10,904.8 10,629.3 10,995.4
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 390.1 426.9 655.0
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 4,368.6 4,535.2 5,252.6
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Asset-backed Securities | Other asset-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 6,146.1 5,667.2 5,087.8
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Redeemable preferred stocks      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 181.2 187.7 202.5
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Redeemable preferred stocks | Financials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 24.5 24.5 43.5
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Redeemable preferred stocks | Utilities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 10.4 10.4 10.5
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Redeemable preferred stocks | Industrials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 146.3 152.8 148.5
Carrying (Reported) Amount, Fair Value Disclosure | Short-term investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities $ 1,326.7 $ 1,789.9 $ 2,524.1