Accumulated Other Comprehensive Income - Changes in AOCI (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			$ 17,038
Other comprehensive income before reclassification, net of income taxes	$ 184	$ 511		$ 777
Amounts reclassified from AOCI, net of income taxes	(62)	17		39
Total other comprehensive income (loss), net of tax	122	528	910	816
Ending Balance	17,542		17,542
Unrealized gains (losses) on securities available for sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	520	(189)	115	(373)
Other comprehensive income before reclassification, net of income taxes	130	291		475
Amounts reclassified from AOCI, net of income taxes	0	0		0
Total other comprehensive income (loss), net of tax	130	291	535	475
Ending Balance	650	102	650	102
Unrealized gains (losses) on derivative financial instruments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	627	49	250	(50)
Other comprehensive income before reclassification, net of income taxes	54	204		284
Amounts reclassified from AOCI, net of income taxes	(68)	15		34
Total other comprehensive income (loss), net of tax	(14)	219	363	318
Ending Balance	613	268	613	268
Foreign currency translation adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	0	(11)	0	(14)
Other comprehensive income before reclassification, net of income taxes	0	1		4
Amounts reclassified from AOCI, net of income taxes	0	0		0
Total other comprehensive income (loss), net of tax	0	1	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	(333)	(379)	(339)	(381)
Other comprehensive income before reclassification, net of income taxes	0	15		14
Amounts reclassified from AOCI, net of income taxes	6	2		5
Total other comprehensive income (loss), net of tax	6	17	12	19
Ending Balance	(327)	(362)	(327)	(362)
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	814	(530)	26	(818)
Total other comprehensive income (loss), net of tax	122		910	816
Ending Balance	$ 936	$ (2)	$ 936	$ (2)