Unaudited Consolidated Quarterly Financial Data (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Income Statements, [Line Items]
Net premium income	$ 4,580,000,000	$ 4,648,000,000	$ 4,665,000,000	$ 4,638,000,000	$ 4,778,000,000	$ 5,022,000,000	$ 4,823,000,000	$ 4,602,000,000	$ 18,531,000,000	$ 19,225,000,000	$ 17,570,000,000
Net investment income	812,000,000	811,000,000	802,000,000	794,000,000	813,000,000	842,000,000	822,000,000	801,000,000	3,220,000,000	3,278,000,000	3,135,000,000
Realized investment gains (losses)	15,000,000	30,000,000	(56,000,000)	(140,000,000)	344,000,000	(164,000,000)	(223,000,000)	30,000,000
Other income (loss)	17,000,000	17,000,000	17,000,000	17,000,000	20,000,000	16,000,000	15,000,000	18,000,000	67,000,000	70,000,000	61,000,000
Total revenues	5,424,000,000	5,506,000,000	5,428,000,000	5,309,000,000	5,955,000,000	5,716,000,000	5,437,000,000	5,451,000,000	21,667,000,000	22,559,000,000	20,872,000,000
Total benefits and expenses	4,425,000,000	4,431,000,000	4,383,000,000	4,411,000,000	4,802,000,000	4,753,000,000	4,603,000,000	4,334,000,000	17,649,000,000	18,492,000,000	17,010,000,000
Earnings before income taxes	999,000,000	1,075,000,000	1,045,000,000	898,000,000	1,153,000,000	963,000,000	834,000,000	1,117,000,000	4,018,000,000	4,067,000,000	3,862,000,000
Total income tax expense	(1,585,000,000)	359,000,000	332,000,000	306,000,000	402,000,000	334,000,000	286,000,000	386,000,000	$ (586,000,000)	$ 1,408,000,000	$ 1,329,000,000
Net earnings	$ 2,584,000,000	$ 716,000,000	$ 713,000,000	$ 592,000,000	$ 751,000,000	$ 629,000,000	$ 548,000,000	$ 731,000,000
Net earnings per basic share	$ 6.59	$ 1.81	$ 1.80	$ 1.48	$ 1.85	$ 1.54	$ 1.33	$ 1.75	$ 11.63	$ 6.46	$ 5.88
Net earnings per diluted share	$ 6.54	$ 1.80	$ 1.79	$ 1.47	$ 1.84	$ 1.53	$ 1.32	$ 1.74	$ 11.54	$ 6.42	$ 5.85
Parent Company
Condensed Income Statements, [Line Items]
Net investment income									$ 30,000,000	$ 18,000,000	$ 22,000,000
Total revenues									331,000,000	213,000,000	321,000,000
Earnings before income taxes									(46,000,000)	(277,000,000)	(231,000,000)
Total income tax expense									$ (23,000,000)	$ (102,000,000)	$ (80,000,000)