Investments - Schedule of Net Change in Unrealized Gains (Losses) Investments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Securities, Available-for-sale [Line Items]
Total net change in unrealized gains (losses) on investments	$ (1,811)	$ 758	$ 224
Fixed Maturity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Total net change in unrealized gains (losses) on investments	$ (1,811)	728	225
Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Total net change in unrealized gains (losses) on investments		32	(2)
Other [Member]
Debt Securities, Available-for-sale [Line Items]
Total net change in unrealized gains (losses) on investments		$ (2)	$ 1