Loans and Allowance for Loan Losses - Schedule of Current, Accruing Past Due, and Non-Accrual Loans on an Amortized Cost Basis by Loan Portfolio Class (Details) - USD ($)
$ in Millions

Loans and Allowance for Loan Losses - Schedule of Current, Accruing Past Due, and Non-Accrual Loans on an Amortized Cost Basis by Loan Portfolio Class (Details) - USD ($) $ in Millions	Mar. 31, 2026	Mar. 31, 2025
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	$ 106	$ 5
Current
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	26	5
Accruing 30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	1	0
Accruing 90 Days or Greater Past Due
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	0	0
Non-accrual
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	79	0
Total commercial and industrial
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	55	5
Total commercial and industrial \| Current
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	16	5
Total commercial and industrial \| Accruing 30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	0	0
Total commercial and industrial \| Accruing 90 Days or Greater Past Due
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	0	0
Total commercial and industrial \| Non-accrual
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	39	0
Total commercial and industrial \| Commercial, financial and agricultural
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	55	5
Total commercial and industrial \| Commercial, financial and agricultural \| Current
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	16	5
Total commercial and industrial \| Commercial, financial and agricultural \| Accruing 30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	0
Total commercial and industrial \| Commercial, financial and agricultural \| Accruing 90 Days or Greater Past Due
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	0	0
Total commercial and industrial \| Commercial, financial and agricultural \| Non-accrual
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	39	$ 0
Total commercial real estate
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	47
Total commercial real estate \| Current
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	9
Total commercial real estate \| Accruing 30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	0
Total commercial real estate \| Accruing 90 Days or Greater Past Due
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	0
Total commercial real estate \| Non-accrual
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	38
Total commercial real estate \| Investment properties
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	47
Total commercial real estate \| Investment properties \| Current
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	9
Total commercial real estate \| Investment properties \| Accruing 30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	0
Total commercial real estate \| Investment properties \| Accruing 90 Days or Greater Past Due
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	0
Total commercial real estate \| Investment properties \| Non-accrual
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	38
Total consumer
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	4
Total consumer \| Current
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	1
Total consumer \| Accruing 30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	1
Total consumer \| Accruing 90 Days or Greater Past Due
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	0
Total consumer \| Non-accrual
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	2
Total consumer \| Consumer mortgages
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	3
Total consumer \| Consumer mortgages \| Current
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	0
Total consumer \| Consumer mortgages \| Accruing 30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	1
Total consumer \| Consumer mortgages \| Accruing 90 Days or Greater Past Due
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	0
Total consumer \| Consumer mortgages \| Non-accrual
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	2
Total consumer \| Other consumer loans
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	1
Total consumer \| Other consumer loans \| Current
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	1
Total consumer \| Other consumer loans \| Accruing 30-89 Days Past Due
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	0
Total consumer \| Other consumer loans \| Accruing 90 Days or Greater Past Due
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	0
Total consumer \| Other consumer loans \| Non-accrual
Financing Receivable, Modified [Line Items]
Amortized cost basis of loans, aging analysis	$ 0