XML 53 R42.htm IDEA: XBRL DOCUMENT v3.23.3
Fair Value - Composition of Investment Portfolio by Major Security Type (Detail) - USD ($)
Sep. 30, 2023
Dec. 31, 2022
Sep. 30, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value $ 61,870,500,000 $ 53,548,300,000 $ 52,330,400,000
Fair value 5,932,100,000 5,717,900,000 5,634,700,000
Available-for-sale Securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 58,387,100,000 49,513,600,000 48,410,700,000
Available-for-sale Securities | Total fixed maturities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 56,591,900,000 46,651,900,000 44,173,100,000
Available-for-sale Securities | Total fixed maturities | Debt Securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 46,617,700,000 36,572,700,000 33,574,700,000
Available-for-sale Securities | Total fixed maturities | Debt Securities | U.S. government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 34,293,800,000 25,167,400,000 22,405,500,000
Available-for-sale Securities | Total fixed maturities | Debt Securities | State and local government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 2,054,300,000 1,977,100,000 1,925,500,000
Available-for-sale Securities | Total fixed maturities | Debt Securities | Foreign government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 15,400,000 15,500,000 15,300,000
Available-for-sale Securities | Total fixed maturities | Debt Securities | Corporate debt securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 10,254,200,000 9,412,700,000 9,228,400,000
Available-for-sale Securities | Total fixed maturities | Asset-backed Securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 9,807,100,000 9,894,900,000 10,416,800,000
Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 515,800,000 666,800,000 723,000,000.0
Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 3,977,700,000 4,663,500,000 5,087,900,000
Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Other asset-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 5,313,600,000 4,564,600,000 4,605,900,000
Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 167,100,000 184,300,000 181,600,000
Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Financials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 21,300,000 40,500,000 39,800,000
Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Utilities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 9,300,000 9,100,000 8,800,000
Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Industrials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 136,500,000 134,700,000 133,000,000.0
Available-for-sale Securities | Short-term investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 1,795,200,000 2,861,700,000 4,237,600,000
Total equity securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 3,483,400,000 4,034,700,000 3,919,700,000
Total equity securities | Nonredeemable preferred stocks      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 868,900,000 1,213,200,000 1,254,400,000
Total equity securities | Nonredeemable preferred stocks | Financials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 818,600,000 1,100,800,000 1,141,400,000
Total equity securities | Nonredeemable preferred stocks | Utilities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 34,500,000 71,200,000 71,300,000
Total equity securities | Nonredeemable preferred stocks | Industrials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 15,800,000 41,200,000 41,700,000
Total equity securities | Common equities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 2,614,500,000 2,821,500,000 2,665,300,000
Total equity securities | Common equities | Common Stocks, excluding Other Risk      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 2,591,900,000 2,801,700,000 2,646,600,000
Total equity securities | Common equities | Other risk investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 22,600,000 19,800,000 18,700,000
Fair Value, Inputs, Level 1      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 38,630,600,000 30,799,700,000 29,292,200,000
Fair value 0 0 0
Fair Value, Inputs, Level 1 | Available-for-sale Securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 36,057,000,000 27,977,300,000 26,634,000,000
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 34,293,800,000 25,176,600,000 22,414,400,000
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Debt Securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 34,293,800,000 25,167,400,000 22,405,500,000
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Debt Securities | U.S. government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 34,293,800,000 25,167,400,000 22,405,500,000
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Debt Securities | State and local government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Debt Securities | Foreign government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Debt Securities | Corporate debt securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Other asset-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 9,200,000 8,900,000
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Financials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Utilities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Industrials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 9,200,000 8,900,000
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Short-term investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 1,763,200,000 2,800,700,000 4,219,600,000
Fair Value, Inputs, Level 1 | Total equity securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 2,573,600,000 2,822,400,000 2,658,200,000
Fair Value, Inputs, Level 1 | Total equity securities | Nonredeemable preferred stocks      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 39,000,000.0 60,900,000
Fair Value, Inputs, Level 1 | Total equity securities | Nonredeemable preferred stocks | Financials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 39,000,000.0 60,900,000
Fair Value, Inputs, Level 1 | Total equity securities | Nonredeemable preferred stocks | Utilities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 0
Fair Value, Inputs, Level 1 | Total equity securities | Nonredeemable preferred stocks | Industrials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 0
Fair Value, Inputs, Level 1 | Total equity securities | Common equities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 2,573,600,000 2,783,400,000 2,597,300,000
Fair Value, Inputs, Level 1 | Total equity securities | Common equities | Common Stocks, excluding Other Risk      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 2,573,600,000 2,783,400,000 2,597,300,000
Fair Value, Inputs, Level 1 | Total equity securities | Common equities | Other risk investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 0
Fair Value, Inputs, Level 2      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 23,135,300,000 22,626,700,000 22,937,600,000
Fair value 5,932,100,000 5,717,900,000 5,634,700,000
Fair Value, Inputs, Level 2 | Available-for-sale Securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 22,330,100,000 21,536,300,000 21,776,700,000
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 22,298,100,000 21,475,300,000 21,758,700,000
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Debt Securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 12,323,900,000 11,405,300,000 11,169,200,000
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Debt Securities | U.S. government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 0
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Debt Securities | State and local government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 2,054,300,000 1,977,100,000 1,925,500,000
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Debt Securities | Foreign government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 15,400,000 15,500,000 15,300,000
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Debt Securities | Corporate debt securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 10,254,200,000 9,412,700,000 9,228,400,000
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 9,807,100,000 9,894,900,000 10,416,800,000
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 515,800,000 666,800,000 723,000,000.0
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 3,977,700,000 4,663,500,000 5,087,900,000
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Other asset-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 5,313,600,000 4,564,600,000 4,605,900,000
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 167,100,000 175,100,000 172,700,000
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Financials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 21,300,000 40,500,000 39,800,000
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Utilities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 9,300,000 9,100,000 8,800,000
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Industrials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 136,500,000 125,500,000 124,100,000
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Short-term investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 32,000,000.0 61,000,000.0 18,000,000.0
Fair Value, Inputs, Level 2 | Total equity securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 805,200,000 1,090,400,000 1,160,900,000
Fair Value, Inputs, Level 2 | Total equity securities | Nonredeemable preferred stocks      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 805,200,000 1,090,400,000 1,111,600,000
Fair Value, Inputs, Level 2 | Total equity securities | Nonredeemable preferred stocks | Financials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 770,700,000 994,400,000 1,015,600,000
Fair Value, Inputs, Level 2 | Total equity securities | Nonredeemable preferred stocks | Utilities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 34,500,000 71,200,000 71,300,000
Fair Value, Inputs, Level 2 | Total equity securities | Nonredeemable preferred stocks | Industrials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 24,800,000 24,700,000
Fair Value, Inputs, Level 2 | Total equity securities | Common equities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 49,300,000
Fair Value, Inputs, Level 2 | Total equity securities | Common equities | Common Stocks, excluding Other Risk      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 49,300,000
Fair Value, Inputs, Level 2 | Total equity securities | Common equities | Other risk investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 0
Fair Value, Inputs, Level 3      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 104,600,000 121,900,000 100,600,000
Fair value 0 0 0
Fair Value, Inputs, Level 3 | Available-for-sale Securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Debt Securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Debt Securities | U.S. government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Debt Securities | State and local government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Debt Securities | Foreign government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Debt Securities | Corporate debt securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Other asset-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Financials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Utilities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Industrials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Short-term investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 0
Fair Value, Inputs, Level 3 | Total equity securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 104,600,000 121,900,000 100,600,000
Fair Value, Inputs, Level 3 | Total equity securities | Nonredeemable preferred stocks      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 63,700,000 83,800,000 81,900,000
Fair Value, Inputs, Level 3 | Total equity securities | Nonredeemable preferred stocks | Financials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 47,900,000 67,400,000 64,900,000
Fair Value, Inputs, Level 3 | Total equity securities | Nonredeemable preferred stocks | Utilities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 0 0 0
Fair Value, Inputs, Level 3 | Total equity securities | Nonredeemable preferred stocks | Industrials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 15,800,000 16,400,000 17,000,000.0
Fair Value, Inputs, Level 3 | Total equity securities | Common equities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 40,900,000 38,100,000 18,700,000
Fair Value, Inputs, Level 3 | Total equity securities | Common equities | Common Stocks, excluding Other Risk      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 18,300,000 18,300,000 0
Fair Value, Inputs, Level 3 | Total equity securities | Common equities | Other risk investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 22,600,000 19,800,000 18,700,000
Carrying (Reported) Amount, Fair Value Disclosure      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 63,925,300,000 55,316,000,000 54,664,600,000
Long-term debt 6,887,600,000 6,388,300,000 6,387,400,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 62,285,800,000 53,125,700,000 52,443,300,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 60,490,600,000 50,264,000,000 48,205,700,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Debt Securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 49,547,500,000 39,093,300,000 36,538,300,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Debt Securities | U.S. government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 36,392,200,000 26,770,700,000 24,260,900,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Debt Securities | State and local government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 2,257,600,000 2,180,000,000 2,140,900,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Debt Securities | Foreign government obligations      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 16,900,000 16,800,000 16,700,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Debt Securities | Corporate debt securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 10,880,800,000 10,125,800,000 10,119,800,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 10,754,100,000 10,968,100,000 11,464,800,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 530,400,000 696,100,000 754,200,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 4,699,500,000 5,446,000,000 5,833,900,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Other asset-backed securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 5,524,200,000 4,826,000,000 4,876,700,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 189,000,000.0 202,600,000 202,600,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Financials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 24,500,000 43,600,000 43,600,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Utilities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 10,400,000 10,500,000 10,500,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Industrials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 154,100,000 148,500,000 148,500,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Short-term investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 1,795,200,000 2,861,700,000 4,237,600,000
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 1,639,500,000 2,190,300,000 2,221,300,000
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Nonredeemable preferred stocks      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 976,900,000 1,364,200,000 1,417,600,000
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Nonredeemable preferred stocks | Financials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 921,900,000 1,244,200,000 1,297,600,000
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Nonredeemable preferred stocks | Utilities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 40,000,000.0 79,900,000 79,900,000
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Nonredeemable preferred stocks | Industrials      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 15,000,000.0 40,100,000 40,100,000
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Common equities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 662,600,000 826,100,000 803,700,000
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Common equities | Common Stocks, excluding Other Risk      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value 640,000,000.0 806,300,000 785,000,000.0
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Common equities | Other risk investments      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, fair value $ 22,600,000 $ 19,800,000 $ 18,700,000