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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Investment income:
Interest	$ 111	$ 112	$ 333	$ 334
Dividends	45	43	131	124
Other	1	1	3	3
Total	157	156	467	461
Less investment expenses	3	3	9	8
Total	154	153	458	453
Gain (Loss) on Sale of Investments			367	156
Investment gains and losses, net:
Investment gains and losses, net	458	7	372	156
Equity securities
Investment income:
Gain (Loss) on Sale of Investments	8	0	17	0
Unrealized Gain (Loss) on Investments	450	0	351	0
Investment gains and losses, net:
Gross realized gains	0	1	0	160
Gross realized losses	0	0	0	(14)
Other Than Temporary Impairment Losses	0	0	0	(3)
Other	(1)	3	(3)	3
Investment gains and losses, net	458	1	368	143
Fixed maturities
Investment gains and losses, net:
Gross realized gains	2	3	9	16
Gross realized losses	(1)	0	(2)	0
Other Than Temporary Impairment Losses	0	0	0	(6)
Investment gains and losses, net	$ 1	$ 3	$ 7	$ 10