Investments - Realized Investment Gains and Losses and Change in Unrealized Investment Gains and Losses and Other Items (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Investment income:
Interest	$ 112	$ 111	$ 334	$ 330
Dividends	43	39	124	117
Other	1	1	3	2
Total	156	151	461	449
Less investment expenses	3	3	8	7
Total	153	148	453	442
Realized investment gains and losses:
Realized investment gains and losses, net	7	56	156	161
Change in unrealized investment gains and losses:
Change in unrealized investment gains and losses	123	41	352	417
change in Income tax provision	(66)	(21)	(189)	(224)
Fixed maturities
Realized investment gains and losses:
Gross realized gains	3	10	16	17
Gross realized losses	0	0	0	(1)
Other-than-temporary impairments	0	0	(6)	(2)
Change in unrealized investment gains and losses:
Change in unrealized investment gains and losses	9	(20)	119	274
Equity securities
Realized investment gains and losses:
Gross realized gains	1	47	160	147
Gross realized losses	0	0	(14)	(1)
Other-than-temporary impairments	0	0	(3)	0
Other	3	(1)	3	1
Change in unrealized investment gains and losses:
Change in unrealized investment gains and losses	$ 180	$ 82	$ 422	$ 367