Accumulated Other Comprehensive Income - Changes in AOCI (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 17,423	$ 17,981
Other comprehensive income before reclassification, net of income taxes	(2,296)	(571)
Amounts reclassified from AOCI, net of income taxes	(47)	(64)
Total other comprehensive income (loss), net of tax	(2,343)	(635)
Ending balance	15,308	17,634
Unrealized gains (losses) on securities available for sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(403)	567
Other comprehensive income before reclassification, net of income taxes	(1,784)	(628)
Amounts reclassified from AOCI, net of income taxes	0	0
Total other comprehensive income (loss), net of tax	(1,784)	(628)
Ending balance	(2,187)	(61)
Unrealized gains (losses) on derivative financial instruments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	88	476
Other comprehensive income before reclassification, net of income taxes	(511)	58
Amounts reclassified from AOCI, net of income taxes	(50)	(68)
Total other comprehensive income (loss), net of tax	(561)	(10)
Ending balance	(473)	466
Net pension and postretirement benefit costs
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(271)	(305)
Other comprehensive income before reclassification, net of income taxes	(1)	(1)
Amounts reclassified from AOCI, net of income taxes	3	4
Total other comprehensive income (loss), net of tax	2	3
Ending balance	(269)	(302)
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(586)	738
Total other comprehensive income (loss), net of tax	(2,343)	(635)
Ending balance	$ (2,929)	$ 103