Loans and Card Member Receivables - Performance (Details) - USD ($) $ in Millions	Mar. 31, 2024	Mar. 31, 2023
Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	$ 3,029	$ 667
30-89 Days Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	235	113
90+ Days Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	79	27
Card Member Loans | Current | Consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	1,635	273
Card Member Loans | Current | Small Business
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	535	119
Card Member Loans | Current | Corporate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	0	0
Card Member Loans | 30-89 Days Past Due | Consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	106	43
Card Member Loans | 30-89 Days Past Due | Small Business
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	51	20
Card Member Loans | 30-89 Days Past Due | Corporate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	0	0
Card Member Loans | 90+ Days Past Due | Consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	39	13
Card Member Loans | 90+ Days Past Due | Small Business
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	17	5
Card Member Loans | 90+ Days Past Due | Corporate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	0	0
Card Member Receivables | Current | Consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	312	100
Card Member Receivables | Current | Small Business
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	468	157
Card Member Receivables | Current | Corporate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	8	7
Card Member Receivables | 30-89 Days Past Due | Consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	23	14
Card Member Receivables | 30-89 Days Past Due | Small Business
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	48	36
Card Member Receivables | 30-89 Days Past Due | Corporate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	1	0
Card Member Receivables | 90+ Days Past Due | Consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	6	3
Card Member Receivables | 90+ Days Past Due | Small Business
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	13	6
Card Member Receivables | 90+ Days Past Due | Corporate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	2	0
Other Loans | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	71	11
Other Loans | 30-89 Days Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	6	0
Other Loans | 90+ Days Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	$ 2	$ 0