Stockholders' Equity and EPS - Reclassification out of AOCI (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gains on investment securities, net	$ 305	$ 35	$ 472	$ 81
Related tax benefit	(173)	(88)	(360)	(137)
Net interest income	728	513	1,388	1,037
Total reclassification adjustment for gains included in net income, net of tax	502	229	1,034	361
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	(11)	(12)	(22)	(57)
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	(5)	(4)	9	5
Net interest income	$ (16)	$ (16)	$ (31)	$ (18)