Derivative Instruments and Hedging Activities - Offsetting Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Derivative liabilities(1)(2)
Derivative Liabilities, Gross Amount	$ 1,268	$ 1,438
Derivative Liability, Offsetting Financial Instruments	(202)	(152)
Derivative Liability, Offsetting Cash Collateral	(460)	(184)
Derivative Liability	606	1,102
Derivative Liability, Securities Collateral Not Netted	0	0
Net Exposure	606	1,102
Repurchase agreements(3)
Gross Amounts	576	992
Securities Sold under Agreements to Repurchase, Asset	0	0
Repurchase Agreement, Collateral, Right To Reclaim Cash, Offset	0	0
Net Amounts as Recognized	576	992
Offsetting Amounts Not Netted, Collateral Pledged	(576)	(992)
Net Exposure	0	0
Securities Sold under Agreements to Repurchase, Fair Value of Collateral	588
Derivative, Collateral, Obligation to Return Cash	91	448
Derivative, Collateral, Obligation to Return Securities	1	16
Derivative, Collateral, Right to Reclaim Cash	$ 966	$ 1,500