Accumulated Other Comprehensive Income - Changes in AOCI (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 17,941	$ 17,542	$ 17,981	$ 17,038
Other comprehensive income before reclassification, net of income taxes	(145)	2	(608)	996
Amounts reclassified from AOCI, net of income taxes	(50)	(71)	(175)	(155)
Total other comprehensive income (loss), net of tax	(195)	(69)	(783)	841
Ending balance	17,510	17,722	17,510	17,722
Unrealized gains (losses) on securities available for sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	124	650	567	115
Other comprehensive income before reclassification, net of income taxes	(174)	(8)	(617)	530
Amounts reclassified from AOCI, net of income taxes	0	0	0	(3)
Total other comprehensive income (loss), net of tax	(174)	(8)	(617)	527
Ending balance	(50)	642	(50)	642
Unrealized gains (losses) on derivative financial instruments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	325	613	476	250
Other comprehensive income before reclassification, net of income taxes	30	11	11	467
Amounts reclassified from AOCI, net of income taxes	(59)	(75)	(191)	(168)
Total other comprehensive income (loss), net of tax	(29)	(64)	(180)	299
Ending balance	296	549	296	549
Net pension and postretirement benefit costs
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(299)	(327)	(305)	(339)
Other comprehensive income before reclassification, net of income taxes	(1)	(1)	(2)	(1)
Amounts reclassified from AOCI, net of income taxes	9	4	16	16
Total other comprehensive income (loss), net of tax	8	3	14	15
Ending balance	(291)	(324)	(291)	(324)
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	150	936	738	26
Total other comprehensive income (loss), net of tax	(195)	(69)	(783)	841
Ending balance	$ (45)	$ 867	$ (45)	$ 867