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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
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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

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