Card Balances and Other Loans - Performance (Details) - USD ($) $ in Millions	Mar. 31, 2026	Mar. 31, 2025
Current
Financing Receivable, Modifications [Line Items]
Account balances	$ 3,056	$ 2,842
30-89 Days Past Due
Financing Receivable, Modifications [Line Items]
Account balances	244	221
90+ Days Past Due
Financing Receivable, Modifications [Line Items]
Account balances	86	77
Card Balances \| Current \| Consumer
Financing Receivable, Modifications [Line Items]
Account balances	1,948	1,812
Card Balances \| Current \| Small Business
Financing Receivable, Modifications [Line Items]
Account balances	1,005	941
Card Balances \| Current \| Corporate
Financing Receivable, Modifications [Line Items]
Account balances	13	8
Card Balances \| 30-89 Days Past Due \| Consumer
Financing Receivable, Modifications [Line Items]
Account balances	130	119
Card Balances \| 30-89 Days Past Due \| Small Business
Financing Receivable, Modifications [Line Items]
Account balances	106	94
Card Balances \| 30-89 Days Past Due \| Corporate
Financing Receivable, Modifications [Line Items]
Account balances	3	3
Card Balances \| 90+ Days Past Due \| Consumer
Financing Receivable, Modifications [Line Items]
Account balances	49	47
Card Balances \| 90+ Days Past Due \| Small Business
Financing Receivable, Modifications [Line Items]
Account balances	31	28
Card Balances \| 90+ Days Past Due \| Corporate
Financing Receivable, Modifications [Line Items]
Account balances	3	1
Other Loans \| Current
Financing Receivable, Modifications [Line Items]
Account balances	89	81
Other Loans \| 30-89 Days Past Due
Financing Receivable, Modifications [Line Items]
Account balances	5	5
Other Loans \| 90+ Days Past Due
Financing Receivable, Modifications [Line Items]
Account balances	$ 2	$ 1