Loans and Related Allowance for Credit Losses (Aging Analysis) (Details) - USD ($) $ in Millions	Mar. 31, 2024	Mar. 31, 2023
Financing Receivable [Line Items]
Financing receivable, aging analysis	$ 2,817	$ 566
Current or Less Than 30 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	2,001	460
30-59 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	20	5
60-89 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	7	3
90 Days or More Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	8	3
Nonperforming Loans
Financing Receivable [Line Items]
Financing receivable, aging analysis	781	95
Total commercial
Financing Receivable [Line Items]
Financing receivable, aging analysis	2,618	497
Total commercial | Current or Less Than 30 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	1,942	442
Total commercial | 30-59 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	16
Total commercial | 60-89 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	2	1
Total commercial | 90 Days or More Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis
Total commercial | Nonperforming Loans
Financing Receivable [Line Items]
Financing receivable, aging analysis	658	54
Total commercial | Commercial and industrial
Financing Receivable [Line Items]
Financing receivable, aging analysis	1,435	224
Total commercial | Commercial and industrial | Current or Less Than 30 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	1,126	193
Total commercial | Commercial and industrial | 30-59 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	16
Total commercial | Commercial and industrial | 60-89 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	2	1
Total commercial | Commercial and industrial | 90 Days or More Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis
Total commercial | Commercial and industrial | Nonperforming Loans
Financing Receivable [Line Items]
Financing receivable, aging analysis	291	30
Total commercial | Commercial real estate
Financing Receivable [Line Items]
Financing receivable, aging analysis	1,183	273
Total commercial | Commercial real estate | Current or Less Than 30 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	816	249
Total commercial | Commercial real estate | Nonperforming Loans
Financing Receivable [Line Items]
Financing receivable, aging analysis	367	24
Total consumer
Financing Receivable [Line Items]
Financing receivable, aging analysis	199	69
Total consumer | Current or Less Than 30 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	59	18
Total consumer | 30-59 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	4	5
Total consumer | 60-89 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	5	2
Total consumer | 90 Days or More Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	8	3
Total consumer | Nonperforming Loans
Financing Receivable [Line Items]
Financing receivable, aging analysis	123	41
Total consumer | Residential real estate
Financing Receivable [Line Items]
Financing receivable, aging analysis	102	49
Total consumer | Residential real estate | Current or Less Than 30 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	8	10
Total consumer | Residential real estate | 30-59 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis		3
Total consumer | Residential real estate | 60-89 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	1
Total consumer | Residential real estate | 90 Days or More Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	1	1
Total consumer | Residential real estate | Nonperforming Loans
Financing Receivable [Line Items]
Financing receivable, aging analysis	92	35
Total consumer | Home equity
Financing Receivable [Line Items]
Financing receivable, aging analysis	31	5
Total consumer | Home equity | Current or Less Than 30 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	2
Total consumer | Home equity | 90 Days or More Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis
Total consumer | Home equity | Nonperforming Loans
Financing Receivable [Line Items]
Financing receivable, aging analysis	29	5
Total consumer | Credit card
Financing Receivable [Line Items]
Financing receivable, aging analysis	60	13
Total consumer | Credit card | Current or Less Than 30 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	44	7
Total consumer | Credit card | 30-59 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	4	2
Total consumer | Credit card | 60-89 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	4	2
Total consumer | Credit card | 90 Days or More Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	7	2
Total consumer | Credit card | Nonperforming Loans
Financing Receivable [Line Items]
Financing receivable, aging analysis	1
Total consumer | Education
Financing Receivable [Line Items]
Financing receivable, aging analysis	5	1
Total consumer | Education | Current or Less Than 30 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	5	1
Total consumer | Education | 30-59 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis
Total consumer | Education | 90 Days or More Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis
Total consumer | Other consumer
Financing Receivable [Line Items]
Financing receivable, aging analysis	1	1
Total consumer | Other consumer | Current or Less Than 30 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis
Total consumer | Other consumer | Nonperforming Loans
Financing Receivable [Line Items]
Financing receivable, aging analysis	$ 1	$ 1