Business Segments - Impacts of Changes to Internal Management Reporting (Details) $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($)
GB
Segment Reporting Information [Line Items]
Segment profit (loss) before tax during the three months ended March 31, 2021:	$ 144
Segment total assets	11,284
Segment total deposits	44,768
MSS
Segment Reporting Information [Line Items]
Segment profit (loss) before tax during the three months ended March 31, 2021:	48
Segment total assets	55,433
Segment total deposits	2,220
GBM Other
Segment Reporting Information [Line Items]
Segment profit (loss) before tax during the three months ended March 31, 2021:	(1)
Segment total assets	43,807
Segment total deposits	1,699
As Previously Reported | GBM
Segment Reporting Information [Line Items]
Segment profit (loss) before tax during the three months ended March 31, 2021:	191
Segment total assets	110,524
Segment total deposits	$ 48,687