Allowance for Credit Losses - Forecasted Economic Scenarios (Details) - USD ($) $ in Millions	3 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020
Commercial Loans [Member] | Commercial Real Estate Portfolio Segment [Member]
Measurement of ECL Measurements [Line Items]
Reduction to the provision for credit losses	$ 24
Exit of Mass Market Retail Banking Business and Remaining Retail Credit Card Portfolio [Member]
Measurement of ECL Measurements [Line Items]
Reduction to the provision for credit losses		$ 101
Reversal or charge-off of allowances for credit losses		$ 157
Forecast, Quarter ended December 31, 2021 [Member]
Measurement of ECL Measurements [Line Items]
Unemployment rate (percent)	4.70%	4.80%	6.20%
GDP growth rate (percent)	6.80%	6.30%	5.30%
Forecast, Quarter ended June 30, 2022 [Member]
Measurement of ECL Measurements [Line Items]
Unemployment rate (percent)	4.30%	4.40%	5.90%
GDP growth rate (percent)	4.90%	4.70%	3.10%
Forecast, Quarter ended December 31, 2022 [Member]
Measurement of ECL Measurements [Line Items]
Unemployment rate (percent)	4.20%	4.10%	5.40%
GDP growth rate (percent)	2.30%	2.30%	1.80%