Loans - Consumer, Excluding Credit Card Loan Portfolio, Residential Real Estate, Payment Status of FDMs and Defaults of FDMs (Details) - Consumer, excluding credit card - Residential real estate - USD ($) $ in Millions	Mar. 31, 2026	Mar. 31, 2025
Financing Receivable, Modified [Line Items]
Amortized cost of FDMs during the last 12 months	$ 972	$ 231
Current
Financing Receivable, Modified [Line Items]
Amortized cost of FDMs during the last 12 months	453	130
30–149 days past due
Financing Receivable, Modified [Line Items]
Amortized cost of FDMs during the last 12 months	58	55
150 or more days past due
Financing Receivable, Modified [Line Items]
Amortized cost of FDMs during the last 12 months	$ 461	$ 46