Loans, Leases, and Allowance for Credit Losses (Summary Aging Analysis, Modified) (Details) - USD ($) $ in Millions	Mar. 31, 2024	Mar. 31, 2023
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	$ 338	$ 97
Current
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	307	80
30-89 days past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	18	17
90+ days past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	13	0
Total past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	31	17
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	86	46
Commercial | Current
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	80	30
Commercial | 30-89 days past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	0	16
Commercial | 90+ days past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	6	0
Commercial | Total past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	6	16
Commercial | Commercial and industrial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	68	36
Commercial | Commercial and industrial | Current
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	62	20
Commercial | Commercial and industrial | 30-89 days past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	0	16
Commercial | Commercial and industrial | 90+ days past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	6	0
Commercial | Commercial and industrial | Total past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	6	16
Commercial | Owner-occupied
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	10	10
Commercial | Owner-occupied | Current
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	10	10
Commercial | Owner-occupied | 30-89 days past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	0	0
Commercial | Owner-occupied | 90+ days past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	0	0
Commercial | Owner-occupied | Total past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	0	0
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	245	49
Commercial real estate | Current
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	222	49
Commercial real estate | 30-89 days past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	18	0
Commercial real estate | 90+ days past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	5	0
Commercial real estate | Total past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	23	0
Commercial real estate | Term
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	220	49
Commercial real estate | Term | Current
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	199	49
Commercial real estate | Term | 30-89 days past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	17	0
Commercial real estate | Term | 90+ days past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	4	0
Commercial real estate | Term | Total past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	21	0
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	7	2
Consumer | Current
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	5	1
Consumer | 30-89 days past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	0	1
Consumer | 90+ days past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	2	0
Consumer | Total past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	2	1
Consumer | 1-4 family residential
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	4	1
Consumer | 1-4 family residential | Current
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	2	0
Consumer | 1-4 family residential | 30-89 days past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	0	1
Consumer | 1-4 family residential | 90+ days past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	2	0
Consumer | 1-4 family residential | Total past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	2	1
Consumer | Bankcard and other revolving plans
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	0	1
Consumer | Bankcard and other revolving plans | Current
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	0	1
Consumer | Bankcard and other revolving plans | 30-89 days past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	0	0
Consumer | Bankcard and other revolving plans | 90+ days past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	0	0
Consumer | Bankcard and other revolving plans | Total past due
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	$ 0	$ 0