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Fair Value - Composition of Investment Portfolio by Major Security Type (Detail) - USD ($) | Sep. 30, 2023 | Dec. 31, 2022 | Sep. 30, 2022
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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

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