Business Segments - Impacts of Changes to Internal Management Reporting (Details) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2021 USD ($)
GB
Segment Reporting Information [Line Items]
Segment profit (loss) before tax during the nine months ended September 30, 2021:	$ 108	$ 420
Segment total assets	10,301	10,301
Segment total deposits	49,465	49,465
MSS
Segment Reporting Information [Line Items]
Segment profit (loss) before tax during the nine months ended September 30, 2021:	14	50
Segment total assets	41,193	41,193
Segment total deposits	1,719	1,719
GBM Other
Segment Reporting Information [Line Items]
Segment profit (loss) before tax during the nine months ended September 30, 2021:	(19)	(21)
Segment total assets	51,049	51,049
Segment total deposits	946	946
As Previously Reported | GBM
Segment Reporting Information [Line Items]
Segment profit (loss) before tax during the nine months ended September 30, 2021:	103	449
Segment total assets	102,543	102,543
Segment total deposits	$ 52,130	$ 52,130