Loans - FDM Disclosure - Modification to Borrowers (Details) - USD ($) $ in Millions	Mar. 31, 2024	Mar. 31, 2023
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	$ 3,032	$ 787
Commitments to lend on loans modified in TDRs	190	26
Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	2,493	527
30-59 days
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	170	89
60-89 days
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	148	34
Greater than 90 days
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	221	137
Total Delinquent Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	539	260
Credit Card:
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	752	188
Credit Card: | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	460	73
Credit Card: | 30-59 days
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	77	41
Credit Card: | 60-89 days
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	66	27
Credit Card: | Greater than 90 days
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	149	47
Credit Card: | Total Delinquent Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	292	115
Credit Card: | Domestic Card
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	646	150
Credit Card: | Domestic Card | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	407	62
Credit Card: | Domestic Card | 30-59 days
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	67	36
Credit Card: | Domestic Card | 60-89 days
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	56	23
Credit Card: | Domestic Card | Greater than 90 days
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	116	29
Credit Card: | Domestic Card | Total Delinquent Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	239	88
Credit Card: | International card businesses
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	106	38
Credit Card: | International card businesses | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	53	11
Credit Card: | International card businesses | 30-59 days
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	10	5
Credit Card: | International card businesses | 60-89 days
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	10	4
Credit Card: | International card businesses | Greater than 90 days
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	33	18
Credit Card: | International card businesses | Total Delinquent Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	53	27
Consumer Banking:
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	802	255
Consumer Banking: | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	635	226
Consumer Banking: | 30-59 days
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	93	18
Consumer Banking: | 60-89 days
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	55	7
Consumer Banking: | Greater than 90 days
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	19	4
Consumer Banking: | Total Delinquent Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	167	29
Consumer Banking: | Auto
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	792	252
Consumer Banking: | Auto | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	626	224
Consumer Banking: | Auto | 30-59 days
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	93	18
Consumer Banking: | Auto | 60-89 days
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	54	7
Consumer Banking: | Auto | Greater than 90 days
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	19	3
Consumer Banking: | Auto | Total Delinquent Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	166	28
Consumer Banking: | Retail banking
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	10	3
Consumer Banking: | Retail banking | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	9	2
Consumer Banking: | Retail banking | 30-59 days
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	0	0
Consumer Banking: | Retail banking | 60-89 days
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	1	0
Consumer Banking: | Retail banking | Greater than 90 days
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	0	1
Consumer Banking: | Retail banking | Total Delinquent Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	1	1
Commercial Banking:
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	1,478	344
Commercial Banking: | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	1,398	228
Commercial Banking: | 30-59 days
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	0	30
Commercial Banking: | 60-89 days
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	27	0
Commercial Banking: | Greater than 90 days
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	53	86
Commercial Banking: | Total Delinquent Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	80	116
Commercial Banking: | Commercial and multifamily real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	578	129
Commercial Banking: | Commercial and multifamily real estate | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	556	99
Commercial Banking: | Commercial and multifamily real estate | 30-59 days
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	0	30
Commercial Banking: | Commercial and multifamily real estate | 60-89 days
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	0	0
Commercial Banking: | Commercial and multifamily real estate | Greater than 90 days
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	22	0
Commercial Banking: | Commercial and multifamily real estate | Total Delinquent Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	22	30
Commercial Banking: | Commercial and industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	900	215
Commercial Banking: | Commercial and industrial | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	842	129
Commercial Banking: | Commercial and industrial | 30-59 days
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	0	0
Commercial Banking: | Commercial and industrial | 60-89 days
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	27	0
Commercial Banking: | Commercial and industrial | Greater than 90 days
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	31	86
Commercial Banking: | Commercial and industrial | Total Delinquent Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Loans	$ 58	$ 86