Unaudited Consolidated Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Income Statements, [Line Items]
Net premium income	$ 4,591	$ 4,636	$ 4,706	$ 4,745	$ 4,580	$ 4,648	$ 4,665	$ 4,638	$ 18,677	$ 18,531	$ 19,225
Net investment income	874	870	862	837	812	811	802	794	3,442	3,220	3,278
Realized investment gains (losses)	(355)	56	3	(134)	15	30	(56)	(140)
Other income (loss)	16	15	18	16	17	17	17	17	69	67	70
Total revenues	5,126	5,577	5,589	5,464	5,424	5,506	5,428	5,309	21,758	21,667	22,559
Total benefits and expenses	4,404	4,431	4,458	4,482	4,425	4,431	4,383	4,411	17,775	17,649	18,492
Earnings before income taxes	722	1,146	1,131	982	999	1,075	1,045	898	3,983	4,018	4,067
Total income tax expense	197	301	299	265	(1,585)	359	332	306	$ 1,063	$ (586)	$ 1,408
Net earnings	$ 525	$ 845	$ 832	$ 717	$ 2,584	$ 716	$ 713	$ 592
Net earnings per basic share	$ 0.69	$ 1.10	$ 1.08	$ 0.92	$ 3.29	$ 0.91	$ 0.90	$ 0.74	$ 3.79	$ 5.81	$ 3.23
Net earnings per diluted share	$ 0.69	$ 1.09	$ 1.07	$ 0.91	$ 3.27	$ 0.90	$ 0.89	$ 0.73	$ 3.77	$ 5.77	$ 3.21
Parent Company
Condensed Income Statements, [Line Items]
Net investment income									$ 69	$ 30	$ 18
Total revenues									247	331	213
Total income tax expense									$ (12)	$ (23)	$ (102)