Derivative Financial Instruments - Cash Flow and Fair Value Hedges (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Cash flow hedges
Derivative [Line Items]
Net unrecognized after-tax gain (loss) on active hedges recorded in AOCI	$ (13)	$ 96
Net unrecognized after-tax gain (loss) on terminated hedges recorded in AOCI (to be recognized in earnings through 2022)	(1)	3
Estimated portion of net after-tax gain (loss) on active and terminated hedges to be reclassified from AOCI into earnings during the next 12 months	$ 0	$ (25)
Maximum length of time entity has hedged portion of variable cash flows for forecasted transactions (in years)	4 years	5 years
Fair Value Hedges
Derivative [Line Items]
Unrecognized pre-tax net gain on terminated hedges (to be recognized as interest primarily through 2019)	$ 88	$ 129
Portion of pre-tax net gain on terminated hedges to be recognized as a change in interest during the next 12 months	$ 44	$ 49