Accumulated Other Comprehensive Income - Changes in AOCI (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			$ 17,038
Other comprehensive income before reclassification, net of income taxes	$ 2	$ 164	996	$ 941
Amounts reclassified from AOCI, net of income taxes	(71)	(3)	(155)	36
Total other comprehensive income (loss), net of tax	(69)	161	841	977
Ending Balance	17,722		17,722
Unrealized gains (losses) on securities available for sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	650	102	115	(373)
Other comprehensive income before reclassification, net of income taxes	(8)	104	530	579
Amounts reclassified from AOCI, net of income taxes	0	(11)	(3)	(11)
Total other comprehensive income (loss), net of tax	(8)	93	527	568
Ending Balance	642	195	642	195
Unrealized gains (losses) on derivative financial instruments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	613	268	250	(50)
Other comprehensive income before reclassification, net of income taxes	11	62	467	346
Amounts reclassified from AOCI, net of income taxes	(75)	6	(168)	40
Total other comprehensive income (loss), net of tax	(64)	68	299	386
Ending Balance	549	336	549	336
Foreign currency translation adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	0	(10)	0	(14)
Other comprehensive income before reclassification, net of income taxes	0	0	0	4
Amounts reclassified from AOCI, net of income taxes	0	0	0	0
Total other comprehensive income (loss), net of tax	0	0	0	4
Ending Balance	0	(10)	0	(10)
Net pension and postretirement benefit costs
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(327)	(362)	(339)	(381)
Other comprehensive income before reclassification, net of income taxes	(1)	(2)	(1)	12
Amounts reclassified from AOCI, net of income taxes	4	2	16	7
Total other comprehensive income (loss), net of tax	3	0	15	19
Ending Balance	(324)	(362)	(324)	(362)
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	936	(2)	26	(818)
Total other comprehensive income (loss), net of tax	(69)		841	977
Ending Balance	$ 867	$ 159	$ 867	$ 159