Fair Value Composition of Investment Portfolio by Major Security Type (Detail) - USD ($)	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 30,921,200,000	$ 27,274,700,000	$ 25,978,400,000
Fair Value	3,959,000,000	3,643,600,000	3,679,200,000
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	14,049,000,000	11,950,400,000	9,788,000,000
Fair Value	0	0	0
Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	16,866,900,000	15,319,000,000	16,190,100,000
Fair Value	3,959,000,000	3,606,500,000	3,564,400,000
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	5,300,000	5,300,000	300,000
Fair Value	0	37,100,000	114,800,000
Total Fixed Maturities And Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		24,405,300,000	22,248,700,000
Total Fixed Maturities And Equity Securities [Member] | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		10,126,000,000	7,340,000,000
Total Fixed Maturities And Equity Securities [Member] | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		14,274,000,000	14,908,400,000
Total Fixed Maturities And Equity Securities [Member] | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		5,300,000	300,000
Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	27,020,400,000
Available-for-sale Securities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	10,829,700,000
Available-for-sale Securities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	16,190,700,000
Available-for-sale Securities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0
Total equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,900,800,000
Total equity securities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,219,300,000
Total equity securities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	676,200,000
Total equity securities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	5,300,000
Total equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	758,600,000	803,800,000	783,100,000
Total equity securities | Nonredeemable preferred stocks | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	77,400,000	80,600,000	85,300,000
Total equity securities | Nonredeemable preferred stocks | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	676,200,000	718,200,000	697,800,000
Total equity securities | Nonredeemable preferred stocks | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	5,000,000	5,000,000	0
Total equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	753,600,000	798,800,000	783,100,000
Total equity securities | Nonredeemable preferred stocks | Financials | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	77,400,000	80,600,000	85,300,000
Total equity securities | Nonredeemable preferred stocks | Financials | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	676,200,000	718,200,000	697,800,000
Total equity securities | Nonredeemable preferred stocks | Financials | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	5,000,000	5,000,000	0
Total equity securities | Nonredeemable preferred stocks | Industrials | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total equity securities | Nonredeemable preferred stocks | Industrials | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total equity securities | Nonredeemable preferred stocks | Industrials | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	5,000,000	5,000,000	0
Total equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,142,200,000	3,399,800,000	3,077,500,000
Total equity securities | Common equities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,141,900,000	3,399,500,000	3,077,200,000
Total equity securities | Common equities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total equity securities | Common equities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	300,000	300,000	300,000
Total equity securities | Common equities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,141,900,000	3,399,500,000	3,077,200,000
Total equity securities | Common equities | Common equities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,141,900,000	3,399,500,000	3,077,200,000
Total equity securities | Common equities | Common equities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total equity securities | Common equities | Common equities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	300,000	300,000	300,000
Total equity securities | Common equities | Other risk investments | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total equity securities | Common equities | Other risk investments | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total equity securities | Common equities | Other risk investments | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	300,000	300,000	300,000
Carrying (Reported) Amount, Fair Value Disclosure
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	29,284,000,000	25,276,900,000	24,179,700,000
Long-term Debt	3,859,500,000	3,306,300,000	3,383,400,000
Carrying (Reported) Amount, Fair Value Disclosure | Total Fixed Maturities And Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		22,407,500,000	20,450,000,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	27,293,000,000
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,991,000,000
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]
Fair Value	677,000,000	698,600,000	666,000,000
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]
Fair Value	672,000,000	693,600,000	666,000,000
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]
Fair Value	5,000,000	5,000,000	0
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,314,000,000	1,499,000,000	1,472,300,000
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Common equities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,313,700,000	1,498,700,000	1,472,000,000
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]
Fair Value	300,000	300,000	300,000
Total fixed maturities | Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	23,789,200,000	20,201,700,000	18,388,100,000
Total fixed maturities | Available-for-sale Securities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	7,875,500,000	6,645,900,000	4,177,500,000
Total fixed maturities | Available-for-sale Securities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	15,913,700,000	13,555,800,000	14,210,600,000
Total fixed maturities | Available-for-sale Securities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	16,863,000,000	13,940,700,000	11,691,400,000
Total fixed maturities | Available-for-sale Securities | Debt Securities [Member] | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	7,865,400,000	6,645,900,000	4,177,500,000
Total fixed maturities | Available-for-sale Securities | Debt Securities [Member] | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	8,997,600,000	7,294,800,000	7,513,900,000
Total fixed maturities | Available-for-sale Securities | Debt Securities [Member] | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	6,697,100,000	6,050,000,000	6,490,000,000
Total fixed maturities | Available-for-sale Securities | Asset-backed Securities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Asset-backed Securities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	6,697,100,000	6,050,000,000	6,490,000,000
Total fixed maturities | Available-for-sale Securities | Asset-backed Securities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | U.S. government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	7,865,400,000	6,645,900,000	4,155,000,000
Total fixed maturities | Available-for-sale Securities | U.S. government obligations | Debt Securities [Member] | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	7,865,400,000	6,645,900,000	4,155,000,000
Total fixed maturities | Available-for-sale Securities | U.S. government obligations | Debt Securities [Member] | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | U.S. government obligations | Debt Securities [Member] | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | State and local government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,667,300,000	2,297,100,000	2,500,100,000
Total fixed maturities | Available-for-sale Securities | State and local government obligations | Debt Securities [Member] | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | State and local government obligations | Debt Securities [Member] | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,667,300,000	2,297,100,000	2,500,100,000
Total fixed maturities | Available-for-sale Securities | State and local government obligations | Debt Securities [Member] | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Foreign government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	22,500,000
Total fixed maturities | Available-for-sale Securities | Foreign government obligations | Debt Securities [Member] | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	22,500,000
Total fixed maturities | Available-for-sale Securities | Foreign government obligations | Debt Securities [Member] | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Foreign government obligations | Debt Securities [Member] | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Corporate debt securities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	7,330,300,000	4,997,700,000	5,013,800,000
Total fixed maturities | Available-for-sale Securities | Corporate debt securities | Debt Securities [Member] | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Corporate debt securities | Debt Securities [Member] | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	7,330,300,000	4,997,700,000	5,013,800,000
Total fixed maturities | Available-for-sale Securities | Corporate debt securities | Debt Securities [Member] | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Residential mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	822,600,000	836,700,000	1,267,600,000
Total fixed maturities | Available-for-sale Securities | Residential mortgage-backed securities | Asset-backed Securities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Residential mortgage-backed securities | Asset-backed Securities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	822,600,000	836,700,000	1,267,600,000
Total fixed maturities | Available-for-sale Securities | Residential mortgage-backed securities | Asset-backed Securities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Commercial mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,696,900,000	2,758,600,000	2,375,000,000
Total fixed maturities | Available-for-sale Securities | Commercial mortgage-backed securities | Asset-backed Securities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Commercial mortgage-backed securities | Asset-backed Securities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,696,900,000	2,758,600,000	2,375,000,000
Total fixed maturities | Available-for-sale Securities | Commercial mortgage-backed securities | Asset-backed Securities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Other asset-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,177,600,000	2,454,700,000	2,847,400,000
Total fixed maturities | Available-for-sale Securities | Other asset-backed securities | Asset-backed Securities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Other asset-backed securities | Asset-backed Securities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,177,600,000	2,454,700,000	2,847,400,000
Total fixed maturities | Available-for-sale Securities | Other asset-backed securities | Asset-backed Securities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	229,100,000	211,000,000	206,700,000
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	10,100,000	0	0
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	219,000,000	211,000,000	206,700,000
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	67,700,000	64,100,000	64,100,000
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Financials | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Financials | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	67,700,000	64,100,000	64,100,000
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Financials | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Public Utility, Equities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	4,600,000	11,400,000	32,200,000
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Public Utility, Equities [Member] | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Public Utility, Equities [Member] | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	4,600,000	11,400,000	32,200,000
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Public Utility, Equities [Member] | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	156,800,000	135,500,000	110,400,000
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Industrials | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	10,100,000	0	0
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Industrials | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	146,700,000	135,500,000	110,400,000
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Industrials | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	24,061,800,000	20,209,900,000	18,311,700,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	17,107,100,000	13,971,600,000	11,653,800,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	6,734,300,000	6,043,400,000	6,468,500,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | U.S. government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	8,005,800,000	6,688,800,000	4,166,400,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | State and local government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,678,900,000	2,285,600,000	2,473,300,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Foreign government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	22,500,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Corporate debt securities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	7,422,400,000	4,997,200,000	4,991,600,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Residential mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	819,600,000	828,800,000	1,260,300,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Commercial mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,725,500,000	2,760,100,000	2,364,900,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Other asset-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,189,200,000	2,454,500,000	2,843,300,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	220,400,000	194,900,000	189,400,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	65,300,000	61,300,000	60,400,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Redeemable preferred stocks | Public Utility, Equities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	4,800,000	10,100,000	30,500,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	150,300,000	123,500,000	98,500,000
Short-term investments | Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,231,200,000	2,869,400,000	3,729,700,000
Short-term investments | Available-for-sale Securities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,954,200,000	1,824,400,000	2,448,000,000
Short-term investments | Available-for-sale Securities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	277,000,000	1,045,000,000	1,281,700,000
Short-term investments | Available-for-sale Securities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Short-term investments | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 3,231,200,000	$ 2,869,400,000	$ 3,729,700,000