Loans and Card Member Receivables - Performance (Details) - USD ($) $ in Millions	Mar. 31, 2025	Mar. 31, 2024
Current
Financing Receivable, Modifications [Line Items]
Account balances	$ 2,842	$ 3,029
30-89 Days Past Due
Financing Receivable, Modifications [Line Items]
Account balances	221	235
90+ Days Past Due
Financing Receivable, Modifications [Line Items]
Account balances	77	79
Card Member Loans | Current | Consumer
Financing Receivable, Modifications [Line Items]
Account balances	1,597	1,635
Card Member Loans | Current | Small Business
Financing Receivable, Modifications [Line Items]
Account balances	596	535
Card Member Loans | Current | Corporate
Financing Receivable, Modifications [Line Items]
Account balances	0	0
Card Member Loans | 30-89 Days Past Due | Consumer
Financing Receivable, Modifications [Line Items]
Account balances	106	106
Card Member Loans | 30-89 Days Past Due | Small Business
Financing Receivable, Modifications [Line Items]
Account balances	59	51
Card Member Loans | 30-89 Days Past Due | Corporate
Financing Receivable, Modifications [Line Items]
Account balances	0	0
Card Member Loans | 90+ Days Past Due | Consumer
Financing Receivable, Modifications [Line Items]
Account balances	42	39
Card Member Loans | 90+ Days Past Due | Small Business
Financing Receivable, Modifications [Line Items]
Account balances	20	17
Card Member Loans | 90+ Days Past Due | Corporate
Financing Receivable, Modifications [Line Items]
Account balances	0	0
Card Member Receivables | Current | Consumer
Financing Receivable, Modifications [Line Items]
Account balances	215	312
Card Member Receivables | Current | Small Business
Financing Receivable, Modifications [Line Items]
Account balances	345	468
Card Member Receivables | Current | Corporate
Financing Receivable, Modifications [Line Items]
Account balances	8	8
Card Member Receivables | 30-89 Days Past Due | Consumer
Financing Receivable, Modifications [Line Items]
Account balances	13	23
Card Member Receivables | 30-89 Days Past Due | Small Business
Financing Receivable, Modifications [Line Items]
Account balances	35	48
Card Member Receivables | 30-89 Days Past Due | Corporate
Financing Receivable, Modifications [Line Items]
Account balances	3	1
Card Member Receivables | 90+ Days Past Due | Consumer
Financing Receivable, Modifications [Line Items]
Account balances	5	6
Card Member Receivables | 90+ Days Past Due | Small Business
Financing Receivable, Modifications [Line Items]
Account balances	8	13
Card Member Receivables | 90+ Days Past Due | Corporate
Financing Receivable, Modifications [Line Items]
Account balances	1	2
Other Loans | Current
Financing Receivable, Modifications [Line Items]
Account balances	81	71
Other Loans | 30-89 Days Past Due
Financing Receivable, Modifications [Line Items]
Account balances	5	6
Other Loans | 90+ Days Past Due
Financing Receivable, Modifications [Line Items]
Account balances	$ 1	$ 2