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Fair Value - Composition of Investment Portfolio by Major Security Type (Detail) - USD ($)
$ in Millions
Jun. 30, 2024
Dec. 31, 2023
Jun. 30, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities $ 68,221.7 $ 62,168.1 $ 55,572.4
Equity securities 4,133.8 3,830.5 3,693.2
Investments, fair value 72,355.5 65,998.6 59,265.6
Fair value 6,165.5 6,431.3 6,224.0
Nonredeemable preferred stocks      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 838.2 902.1 985.1
Nonredeemable preferred stocks | Financials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 785.5 851.9 896.6
Nonredeemable preferred stocks | Utilities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 37.5 35.4 72.7
Nonredeemable preferred stocks | Industrials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 15.2 14.8 15.8
Common equities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 3,295.6 2,928.4 2,708.1
Common equities | Common Stocks, excluding Other Risk      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 3,271.7 2,907.8 2,686.2
Common equities | Other risk investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 23.9 20.6 21.9
Total fixed maturities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 67,488.3 60,378.2 54,078.1
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 40,894.0 36,869.4 31,600.5
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 2,202.0 2,202.8 2,154.7
Total fixed maturities | Foreign government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 15.8 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 12,974.6 11,183.7 10,304.6
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 975.7 417.2 562.7
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 3,970.0 3,939.7 4,265.5
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 6,284.7 5,575.4 5,017.6
Total fixed maturities | Debt Securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 56,086.4 50,272.2 44,075.6
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 40,894.0 36,869.4 31,600.5
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 2,202.0 2,202.8 2,154.7
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 15.8 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 12,974.6 11,183.7 10,304.6
Total fixed maturities | Asset-backed Securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 11,230.4 9,932.3 9,845.8
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 975.7 417.2 562.7
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 3,970.0 3,939.7 4,265.5
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 6,284.7 5,575.4 5,017.6
Total fixed maturities | Redeemable preferred stocks      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 171.5 173.7 156.7
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 23.1 23.1 22.0
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 13.4 9.4 9.3
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 135.0 141.2 125.4
Short-term investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 733.4 1,789.9 1,494.3
Fair Value, Inputs, Level 1      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 41,627.4 38,626.4 33,062.7
Equity securities 3,249.2 2,885.3 2,683.7
Investments, fair value 44,876.6 41,511.7 35,746.4
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 15.8
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 15.8
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 3,249.2 2,885.3 2,667.9
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 3,249.2 2,885.3 2,667.9
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 40,894.0 36,869.4 31,600.5
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 40,894.0 36,869.4 31,600.5
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 40,894.0 36,869.4 31,600.5
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 0.0
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 0.0
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 733.4 1,757.0 1,462.2
Fair Value, Inputs, Level 2      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total available-for-sale securities 26,591.3 23,538.7 22,509.7
Equity securities 786.3 838.1 897.5
Investments, fair value 27,377.6 24,376.8 23,407.2
Fair value 6,165.5 6,431.3 6,224.0
Fair Value, Inputs, Level 2 | Nonredeemable preferred stocks      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 786.3 838.1 897.5
Fair Value, Inputs, Level 2 | Nonredeemable preferred stocks | Financials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 748.8 802.7 824.8
Fair Value, Inputs, Level 2 | Nonredeemable preferred stocks | Utilities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 37.5 35.4 72.7
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 26,591.3 23,505.8 22,477.6
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 15,189.4 13,399.8 12,475.1
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 2,202.0 2,202.8 2,154.7
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 15.8 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 12,971.6 11,180.7 10,304.6
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 11,230.4 9,932.3 9,845.8
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 975.7 417.2 562.7
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 3,970.0 3,939.7 4,265.5
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 6,284.7 5,575.4 5,017.6
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 171.5 173.7 156.7
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 23.1 23.1 22.0
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 13.4 9.4 9.3
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 135.0 141.2 125.4
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 32.1
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 98.3 107.1 112.0
Investments, fair value 101.3 110.1 112.0
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 51.9 64.0 71.8
Fair Value, Inputs, Level 3 | Nonredeemable preferred stocks | Financials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 36.7 49.2 56.0
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 15.2 14.8 15.8
Fair Value, Inputs, Level 3 | Common equities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 46.4 43.1 40.2
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 23.9 20.6 21.9
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 70,401.6 64,231.8 58,979.0
Equity securities 1,594.7 1,683.1 1,769.1
Investments, fair value 71,996.3 65,914.9 60,748.1
Long-term debt 6,890.7 6,888.6 6,886.5
Carrying (Reported) Amount, Fair Value Disclosure | Nonredeemable preferred stocks      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity securities 886.9 977.1 1,107.1
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 831.9 922.1 1,012.1
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 707.8 706.0 662.0
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 683.9 685.4 640.1
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 23.9 20.6 21.9
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 69,668.2 62,441.9 57,484.7
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 57,685.4 51,624.9 46,497.2
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 42,063.1 37,823.2 33,277.1
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 2,345.6 2,338.4 2,336.5
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 16.7 17.3 17.2
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 13,260.0 11,446.0 10,866.4
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 11,804.0 10,629.3 10,813.5
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 980.4 426.9 580.7
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 4,457.2 4,535.2 4,982.2
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 6,366.4 5,667.2 5,250.6
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 178.8 187.7 174.0
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 24.5
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 13.9 10.4 10.4
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 140.4 152.8 139.1
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 $ 733.4 $ 1,789.9 $ 1,494.3