Stockholders' Equity and EPS - Reclassification out of AOCI (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gains on investment securities, net	$ 189	$ 190	$ 661	$ 271
Related tax benefit	(149)	(162)	(509)	(300)
Net interest income	852	528	2,240	1,565
Total reclassification adjustment for gains included in net income, net of tax	365	442	1,399	803
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassification adjustment for gains included in net income, net of tax	(12)	(11)	(34)	(69)
Accumulated Net Investment Gain (Loss) Attributable to Parent | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gains on investment securities, net	0	0	0	(61)
Related tax benefit	0	0	0	17
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Related tax benefit	(4)	(5)	13	9
Net interest income	$ (16)	$ (16)	$ (47)	$ (34)