Basis of Presentation - Effects on Retained Earnings and AOCI (Details) - USD ($) $ in Millions	Jan. 01, 2018	Jan. 01, 2017	Jan. 01, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase/(decrease)			$ 154
Retained earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase/(decrease)	$ (183)	$ 0	(154)
AOCI
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase/(decrease)	88	$ 0	$ 154
Premium amortization on purchased callable debt securities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase/(decrease)	261
Premium amortization on purchased callable debt securities | Retained earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase/(decrease)	(505)
Premium amortization on purchased callable debt securities | AOCI
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase/(decrease)	261
Hedge accounting
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase/(decrease)	115
Hedge accounting | Retained earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase/(decrease)	34
Hedge accounting | AOCI
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase/(decrease)	115
Reclassification of certain tax effects from AOCI
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase/(decrease)	(288)
Reclassification of certain tax effects from AOCI | Retained earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase/(decrease)	288
Reclassification of certain tax effects from AOCI | AOCI
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase/(decrease)	$ (288)