Credit Losses Recognized in Net Income (Loss) on Debt Securities (Detail) - Debt Securities - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Cumulative credit losses, beginning balance	$ 42	$ 64	$ 83
Other-than-temporary impairments not previously recognized	0	1	0
Securities sold, paid down or disposed	(10)	(23)	(19)
Cumulative credit losses, ending balance	$ 32	$ 42	$ 64