Accumulated Other Comprehensive Income - Changes in AOCI (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 15,596
Other comprehensive income before reclassification, net of income taxes	860	$ (80)
Amounts reclassified from accumulated other comprehensive income, net of income taxes	(16)	54
Amounts reclassified from accumulated other comprehensive income resulting from new federal corporate income tax rate		(13)
Total other comprehensive income (loss), net of tax	844	(39)	$ (238)
Ending Balance	17,038	15,596
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	503	(62)
Amounts reclassified from accumulated other comprehensive income, net of income taxes	(15)	0
Amounts reclassified from accumulated other comprehensive income resulting from new federal corporate income tax rate		0
Total other comprehensive income (loss), net of tax	488	(62)
Ending Balance	115	(373)	(311)
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(50)	(86)
Other comprehensive income before reclassification, net of income taxes	335	7
Amounts reclassified from accumulated other comprehensive income, net of income taxes	(35)	29
Amounts reclassified from accumulated other comprehensive income resulting from new federal corporate income tax rate		0
Total other comprehensive income (loss), net of tax	300	36
Ending Balance	250	(50)	(86)
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	(10)
Amounts reclassified from accumulated other comprehensive income, net of income taxes	11	0
Amounts reclassified from accumulated other comprehensive income resulting from new federal corporate income tax rate		(13)
Total other comprehensive income (loss), net of tax	14	(23)
Ending Balance	0	(14)	9
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	19	(15)
Amounts reclassified from accumulated other comprehensive income, net of income taxes	23	25
Amounts reclassified from accumulated other comprehensive income resulting from new federal corporate income tax rate		0
Total other comprehensive income (loss), net of tax	42	10
Ending Balance	(339)	(381)	(391)
AOCI Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(818)	(779)	(541)
Total other comprehensive income (loss), net of tax	844	(39)	(238)
Ending Balance	$ 26	$ (818)	$ (779)