Accumulated Other Comprehensive Income - Changes in AOCI (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 15,596
Other comprehensive income before reclassification, net of income taxes	266	$ (217)
Amounts reclassified from AOCI, net of income taxes	22	5
Total other comprehensive income (loss), net of tax	288	(212)
Ending Balance	15,926
Unrealized Gains (Losses) on Securities Available for Sale [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(373)	(311)
Other comprehensive income before reclassification, net of income taxes	184	(150)
Amounts reclassified from AOCI, net of income taxes	0	0
Total other comprehensive income (loss), net of tax	184	(150)
Ending Balance	(189)	(461)
Unrealized Gains (Losses) on Derivative Financial Instruments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(50)	(86)
Other comprehensive income before reclassification, net of income taxes	80	(65)
Amounts reclassified from AOCI, net of income taxes	19	2
Total other comprehensive income (loss), net of tax	99	(63)
Ending Balance	49	(149)
Foreign Currency Translation Adjustment [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(14)	9
Other comprehensive income before reclassification, net of income taxes	3	(2)
Amounts reclassified from AOCI, net of income taxes	0	0
Total other comprehensive income (loss), net of tax	3	(2)
Ending Balance	(11)	7
Net Pension and Postretirement Benefit Costs [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(381)	(391)
Other comprehensive income before reclassification, net of income taxes	(1)	0
Amounts reclassified from AOCI, net of income taxes	3	3
Total other comprehensive income (loss), net of tax	2	3
Ending Balance	(379)	(388)
AOCI Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(818)	(779)
Total other comprehensive income (loss), net of tax	288	(212)
Ending Balance	$ (530)	$ (991)