Derivative Instruments - Summary of Effective Portion of Gains (Losses) Recognized in Other Comprehensive Income (Loss) and Gains (Losses) Reclassified from Other Comprehensive Income (Loss) into Earnings (Detail) - Asset and Liability Management Positions [Member] - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Cash Flow Hedges [Member] | Interest Rate Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Losses) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net	$ 25	$ (23)	$ 89	$ (19)
Derivative Instruments, Gains (Losses) Reclassified from Other Comprehensive Income (Loss) into Earnings	1	(6)	(1)	(15)
Net Investment Hedges [Member] | Foreign Exchange Forward Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Losses) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net	12	(41)	28	(48)
Net Investment Hedges [Member] | Non Derivative Debt Instruments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Losses) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net	$ 50	$ (11)	$ 16	$ (11)