Investments - Schedule of Net Change in Unrealized Gains (Losses) on Available-for-Sale Investments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Total net change in unrealized gains (losses) on available-for-sale investments	$ 758	$ 224	$ (1,119)
Fixed Maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total net change in unrealized gains (losses) on available-for-sale investments	728	225	(1,114)
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total net change in unrealized gains (losses) on available-for-sale investments	32	(2)	(6)
Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Total net change in unrealized gains (losses) on available-for-sale investments	$ (2)	$ 1	$ 1