Derivative Instruments and Hedging Activities - Offsetting Assets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative, Collateral, Obligation to Return Cash	$ 91	$ 448
Derivative assets(1)(2)
Derivative Assets, Gross Amount	1,040	1,494
Derivative Asset, Offsetting Financial Instruments	(202)	(152)
Derivative Assets, Offsetting Cash Collateral	(73)	(387)
Derivative Assets	765	955
Derivative Asset, Securities Not Netted	0	(11)
Net Exposure	765	944
Derivative, Collateral, Obligation to Return Securities	1	16
Derivative, Collateral, Right to Reclaim Cash	$ 966	$ 1,500