Gains and Losses Included in Net Income (Loss) from Assets Measured at Fair Value (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair value of financial instruments [Abstract]
Total realized and unrealized gains (losses) included in net income (loss), assets	$ 89	$ (26)	$ 10
Total gains (losses) included in net income (loss) attributable to assets still held, assets	51	17	24
Net Investment Income
Fair value of financial instruments [Abstract]
Total realized and unrealized gains (losses) included in net income (loss), assets	26	44	42
Total gains (losses) included in net income (loss) attributable to assets still held, assets	22	30	33
Net Investment (Gains) Losses
Fair value of financial instruments [Abstract]
Total realized and unrealized gains (losses) included in net income (loss), assets	63	(70)	(32)
Total gains (losses) included in net income (loss) attributable to assets still held, assets	$ 29	$ (13)	$ (9)