Loans and Card Member Receivables - Performance (Details) - USD ($) $ in Millions	Sep. 30, 2025	Sep. 30, 2024
Current
Financing Receivable, Modifications [Line Items]
Account balances	$ 2,891	$ 2,939
30-89 Days Past Due
Financing Receivable, Modifications [Line Items]
Account balances	224	252
90+ Days Past Due
Financing Receivable, Modifications [Line Items]
Account balances	81	95
Card Member Loans | Current | Consumer
Financing Receivable, Modifications [Line Items]
Account balances	1,669	1,647
Card Member Loans | Current | Small Business
Financing Receivable, Modifications [Line Items]
Account balances	631	556
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	114	124
Card Member Loans | 30-89 Days Past Due | Small Business
Financing Receivable, Modifications [Line Items]
Account balances	59	59
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	48
Card Member Loans | 90+ Days Past Due | Small Business
Financing Receivable, Modifications [Line Items]
Account balances	22	23
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	194	261
Card Member Receivables | Current | Small Business
Financing Receivable, Modifications [Line Items]
Account balances	306	386
Card Member Receivables | Current | Corporate
Financing Receivable, Modifications [Line Items]
Account balances	11	11
Card Member Receivables | 30-89 Days Past Due | Consumer
Financing Receivable, Modifications [Line Items]
Account balances	14	19
Card Member Receivables | 30-89 Days Past Due | Small Business
Financing Receivable, Modifications [Line Items]
Account balances	28	41
Card Member Receivables | 30-89 Days Past Due | Corporate
Financing Receivable, Modifications [Line Items]
Account balances	4	3
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	9	14
Card Member Receivables | 90+ Days Past Due | Corporate
Financing Receivable, Modifications [Line Items]
Account balances	2	2
Other Loans | Current
Financing Receivable, Modifications [Line Items]
Account balances	80	78
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