Loans and Card Member Receivables - Performance (Details) - USD ($) $ in Millions	Sep. 30, 2024	Sep. 30, 2023
Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	$ 2,939	$ 2,096
30-89 Days Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	252	176
90+ Days Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	95	48
Card Member Loans | Current | Consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	1,647	1,020
Card Member Loans | Current | Small Business
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	556	366
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	124	73
Card Member Loans | 30-89 Days Past Due | Small Business
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	59	34
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	48	20
Card Member Loans | 90+ Days Past Due | Small Business
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	23	11
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	261	257
Card Member Receivables | Current | Small Business
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	386	402
Card Member Receivables | Current | Corporate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	11	9
Card Member Receivables | 30-89 Days Past Due | Consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	19	20
Card Member Receivables | 30-89 Days Past Due | Small Business
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	41	43
Card Member Receivables | 30-89 Days Past Due | Corporate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	3	2
Card Member Receivables | 90+ Days Past Due | Consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	6	5
Card Member Receivables | 90+ Days Past Due | Small Business
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	14	10
Card Member Receivables | 90+ Days Past Due | Corporate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	2	1
Other Loans | Current
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	78	42
Other Loans | 30-89 Days Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	6	4
Other Loans | 90+ Days Past Due
Financing Receivable, Troubled Debt Restructuring [Line Items]
Account balances	$ 2	$ 1