Business Segments - Impacts of Changes to Internal Management Reporting (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
GB
Segment Reporting Information [Line Items]
Segment profit (loss) before tax:	$ 209	$ 544
Segment total assets:	11,473	16,002
Segment total deposits:	37,320	24,596
MSS
Segment Reporting Information [Line Items]
Segment profit (loss) before tax:	194	132
Segment total assets:	67,293	71,742
Segment total deposits:	3,417	2,956
GBM Other
Segment Reporting Information [Line Items]
Segment profit (loss) before tax:	(16)	(105)
Segment total assets:	53,597	38,966
Segment total deposits:	3,931	1,498
As Previously Reported | GBM
Segment Reporting Information [Line Items]
Segment profit (loss) before tax:	387	571
Segment total assets:	132,363	126,710
Segment total deposits:	$ 44,668	$ 29,050