Investments - Realized Investment Gains and Losses and Change in Unrealized Investment Gains and Losses and Other Items (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Investment income:
Interest	$ 112	$ 111	$ 222	$ 222
Dividends	44	42	86	81
Other	1	1	2	2
Total	157	154	310	305
Less investment expenses	3	3	6	5
Total	154	151	304	300
Gain (Loss) on Sale of Investments			(90)	149
Investment gains and losses, net:
Investment gains and losses, net	105	(11)	(86)	149
Equity securities
Investment income:
Gain (Loss) on Sale of Investments	4	0	7	0
Unrealized Gain (Loss) on Investments	101	0	(97)	0
Investment gains and losses, net:
Gross realized gains	0	6	0	159
Gross realized losses	0	(10)	0	(14)
Other Than Temporary Impairment Losses	0	(3)	0	(3)
Other	(2)	(1)	(2)	0
Investment gains and losses, net	105	(7)	(90)	142
Fixed maturities
Investment gains and losses, net:
Gross realized gains	3	3	7	13
Gross realized losses	(1)	0	(1)	0
Other Than Temporary Impairment Losses	0	(6)	0	(6)
Investment gains and losses, net	$ 2	$ (3)	$ 6	$ 7