Investments - Investment Portfolio by Major Security Type (Detail) - USD ($)	Jun. 30, 2022	Dec. 31, 2021	Jun. 30, 2021
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 50,640,600,000	$ 44,736,800,000	$ 44,088,600,000
Gross Unrealized Gains	7,700,000	387,400,000	740,400,000
Gross Unrealized Losses	(2,784,100,000)	(316,000,000.0)	(101,600,000)
Net Holding Period Gains (Losses)	(79,900,000)	7,500,000	14,200,000
Total available-for-sale securities	$ 47,784,300,000	$ 44,815,700,000	$ 44,741,600,000
% of Total Fair Value	92.00%	87.00%	87.80%
Equity securities, at Fair Value
Cost	$ 2,306,100,000	$ 2,835,900,000	$ 2,749,600,000
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Net Holding Period Gains (Losses)	1,839,100,000	3,862,500,000	3,451,600,000
Fair Value	$ 4,145,200,000	$ 6,698,400,000	$ 6,201,200,000
% of Total Fair Value	8.00%	13.00%	12.20%
Debt and Equity Securities, at Fair Value
Cost	$ 52,946,700,000	$ 47,572,700,000	$ 46,838,200,000
Gross Unrealized Gains	7,700,000	387,400,000	740,400,000
Gross Unrealized Losses	(2,784,100,000)	(316,000,000.0)	(101,600,000)
Net Holding Period Gains (Losses)	1,759,200,000	3,870,000,000	3,465,800,000
Fair Value	$ 51,929,500,000	$ 51,514,100,000	$ 50,942,800,000
% of Total Fair Value	100.00%	100.00%	100.00%
Unsettled security transactions, payable	$ 0	$ 143,400,000	$ 412,100,000
Total fair value of non-insurance subsidiary portfolio	4,900,000,000	4,200,000,000	3,300,000,000
Reclassification, Equity Conversion			52,500,000
Reclassification, Equity Conversion, Cost			25,000,000
Nonredeemable preferred stocks
Equity securities, at Fair Value
Cost	1,522,500,000	1,571,800,000	1,511,200,000
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Net Holding Period Gains (Losses)	(162,000,000.0)	68,100,000	98,600,000
Fair Value	$ 1,360,500,000	$ 1,639,900,000	$ 1,609,800,000
% of Total Fair Value	2.60%	3.20%	3.20%
Common equities
Equity securities, at Fair Value
Cost	$ 783,600,000	$ 1,264,100,000	$ 1,238,400,000
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Net Holding Period Gains (Losses)	2,001,100,000	3,794,400,000	3,353,000,000
Fair Value	$ 2,784,700,000	$ 5,058,500,000	$ 4,591,400,000
% of Total Fair Value	5.40%	9.80%	9.00%
Total fixed maturities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 46,028,800,000	$ 43,794,200,000	$ 42,378,000,000
Gross Unrealized Gains	7,700,000	387,400,000	740,400,000
Gross Unrealized Losses	(2,784,100,000)	(316,000,000.0)	(101,600,000)
Net Holding Period Gains (Losses)	(79,900,000)	7,500,000	14,200,000
Total available-for-sale securities	$ 43,172,500,000	$ 43,873,100,000	$ 43,031,000,000
% of Total Fair Value	83.10%	85.20%	84.50%
Total fixed maturities | U.S. government obligations
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 19,832,300,000	$ 18,586,100,000	$ 19,339,300,000
Gross Unrealized Gains	3,700,000	92,900,000	171,300,000
Gross Unrealized Losses	(1,116,800,000)	(190,800,000)	(72,900,000)
Net Holding Period Gains (Losses)	0	0	0
Total available-for-sale securities	$ 18,719,200,000	$ 18,488,200,000	$ 19,437,700,000
% of Total Fair Value	36.00%	35.90%	38.10%
Total fixed maturities | State and local government obligations
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 2,281,900,000	$ 2,162,600,000	$ 2,379,100,000
Gross Unrealized Gains	600,000	36,700,000	64,700,000
Gross Unrealized Losses	(147,100,000)	(14,000,000.0)	(3,300,000)
Net Holding Period Gains (Losses)	0	0	0
Total available-for-sale securities	$ 2,135,400,000	$ 2,185,300,000	$ 2,440,500,000
% of Total Fair Value	4.10%	4.20%	4.80%
Total fixed maturities | Foreign government obligations
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 17,700,000	$ 17,900,000	$ 12,700,000
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	(1,300,000)	0	0
Net Holding Period Gains (Losses)	0	0	0
Total available-for-sale securities	$ 16,400,000	$ 17,900,000	$ 12,700,000
% of Total Fair Value	0.10%	0.10%	0.10%
Total fixed maturities | Corporate debt securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 10,869,600,000	$ 10,526,200,000	$ 10,314,800,000
Gross Unrealized Gains	1,600,000	202,600,000	372,100,000
Gross Unrealized Losses	(655,000,000.0)	(33,400,000)	(9,900,000)
Net Holding Period Gains (Losses)	(48,400,000)	(3,300,000)	1,200,000
Total available-for-sale securities	$ 10,167,800,000	$ 10,692,100,000	$ 10,678,200,000
% of Total Fair Value	19.60%	20.70%	20.90%
Total fixed maturities | Residential mortgage-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 826,500,000	$ 787,700,000	$ 666,900,000
Gross Unrealized Gains	500,000	2,300,000	4,600,000
Gross Unrealized Losses	(10,700,000)	(600,000)	(1,100,000)
Net Holding Period Gains (Losses)	(17,000,000.0)	600,000	900,000
Total available-for-sale securities	$ 799,300,000	$ 790,000,000.0	$ 671,300,000
% of Total Fair Value	1.50%	1.50%	1.30%
Total fixed maturities | Commercial mortgage-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 6,739,400,000	$ 6,561,000,000	$ 5,628,200,000
Gross Unrealized Gains	1,200,000	38,900,000	91,400,000
Gross Unrealized Losses	(646,000,000.0)	(64,300,000)	(11,500,000)
Net Holding Period Gains (Losses)	0	0	0
Total available-for-sale securities	$ 6,094,600,000	$ 6,535,600,000	$ 5,708,100,000
% of Total Fair Value	11.70%	12.70%	11.20%
Total fixed maturities | Other asset-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 5,239,400,000	$ 4,981,800,000	$ 3,865,900,000
Gross Unrealized Gains	100,000	13,300,000	34,600,000
Gross Unrealized Losses	(201,700,000)	(12,400,000)	(1,500,000)
Net Holding Period Gains (Losses)	(1,800,000)	(400,000)	0
Total available-for-sale securities	$ 5,036,000,000	$ 4,982,300,000	$ 3,899,000,000
% of Total Fair Value	9.70%	9.70%	7.70%
Total fixed maturities | Redeemable preferred stocks
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 222,000,000.0	$ 170,900,000	$ 171,100,000
Gross Unrealized Gains	0	700,000	1,700,000
Gross Unrealized Losses	(5,500,000)	(500,000)	(1,400,000)
Net Holding Period Gains (Losses)	(12,700,000)	10,600,000	12,100,000
Total available-for-sale securities	$ 203,800,000	$ 181,700,000	$ 183,500,000
% of Total Fair Value	0.40%	0.40%	0.40%
Short-term investments
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 4,611,800,000	$ 942,600,000	$ 1,710,600,000
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Net Holding Period Gains (Losses)	0	0	0
Total available-for-sale securities	$ 4,611,800,000	$ 942,600,000	$ 1,710,600,000
% of Total Fair Value	8.90%	1.80%	3.30%