Accumulated Other Comprehensive Income - Changes in AOCI (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			$ 17,981
Other comprehensive income before reclassification, net of income taxes	$ 107	$ 184	(464)	$ 994
Amounts reclassified from AOCI, net of income taxes	(60)	(62)	(124)	(84)
Total other comprehensive income (loss), net of tax	47	122	(588)	910
Ending Balance	17,941		17,941
Unrealized gains (losses) on securities available for sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(61)	520	567	115
Other comprehensive income before reclassification, net of income taxes	185	130	(443)	538
Amounts reclassified from AOCI, net of income taxes	0	0	0	(3)
Total other comprehensive income (loss), net of tax	185	130	(443)	535
Ending Balance	124	650	124	650
Unrealized gains (losses) on derivative financial instruments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	466	627	476	250
Other comprehensive income before reclassification, net of income taxes	(78)	54	(20)	456
Amounts reclassified from AOCI, net of income taxes	(63)	(68)	(131)	(93)
Total other comprehensive income (loss), net of tax	(141)	(14)	(151)	363
Ending Balance	325	613	325	613
Foreign currency translation adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	0	0	0	0
Other comprehensive income before reclassification, net of income taxes	0	0	0	0
Amounts reclassified from AOCI, net of income taxes	0	0	0	0
Total other comprehensive income (loss), net of tax	0	0	0	0
Ending Balance	0	0	0	0
Net pension and postretirement benefit costs
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(302)	(333)	(305)	(339)
Other comprehensive income before reclassification, net of income taxes	0	0	(1)	0
Amounts reclassified from AOCI, net of income taxes	3	6	7	12
Total other comprehensive income (loss), net of tax	3	6	6	12
Ending Balance	(299)	(327)	(299)	(327)
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	103	814	738	26
Total other comprehensive income (loss), net of tax	47		(588)	910
Ending Balance	$ 150	$ 936	$ 150	$ 936