Loans and Related Allowance for Credit Losses (Aging Analysis) (Details) - USD ($) $ in Millions	Mar. 31, 2025	Mar. 31, 2024
Financing Receivable [Line Items]
Financing receivable, aging analysis	$ 2,824	$ 2,817
Current or Less Than 30 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	2,032	2,001
30-59 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	21	20
60-89 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	4	7
90 Days or More Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	7	8
Nonperforming Loans
Financing Receivable [Line Items]
Financing receivable, aging analysis	760	781
Commercial Portfolio Segment
Financing Receivable [Line Items]
Financing receivable, aging analysis	2,624	2,618
Commercial Portfolio Segment | Current or Less Than 30 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	1,974	1,942
Commercial Portfolio Segment | 30-59 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	17	16
Commercial Portfolio Segment | 60-89 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	0	2
Commercial Portfolio Segment | 90 Days or More Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	0	0
Commercial Portfolio Segment | Nonperforming Loans
Financing Receivable [Line Items]
Financing receivable, aging analysis	633	658
Commercial Portfolio Segment | Commercial and industrial
Financing Receivable [Line Items]
Financing receivable, aging analysis	1,387	1,435
Commercial Portfolio Segment | Commercial and industrial | Current or Less Than 30 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	1,182	1,126
Commercial Portfolio Segment | Commercial and industrial | 30-59 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	16	16
Commercial Portfolio Segment | Commercial and industrial | 60-89 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	0	2
Commercial Portfolio Segment | Commercial and industrial | 90 Days or More Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	0	0
Commercial Portfolio Segment | Commercial and industrial | Nonperforming Loans
Financing Receivable [Line Items]
Financing receivable, aging analysis	189	291
Commercial Portfolio Segment | Commercial real estate
Financing Receivable [Line Items]
Financing receivable, aging analysis	1,235	1,183
Commercial Portfolio Segment | Commercial real estate | Current or Less Than 30 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	790	816
Commercial Portfolio Segment | Commercial real estate | 30-59 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	1	0
Commercial Portfolio Segment | Commercial real estate | 60-89 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	0	0
Commercial Portfolio Segment | Commercial real estate | 90 Days or More Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	0	0
Commercial Portfolio Segment | Commercial real estate | Nonperforming Loans
Financing Receivable [Line Items]
Financing receivable, aging analysis	444	367
Commercial Portfolio Segment | Equipment lease financing
Financing Receivable [Line Items]
Financing receivable, aging analysis	2
Commercial Portfolio Segment | Equipment lease financing | Current or Less Than 30 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	2
Commercial Portfolio Segment | Equipment lease financing | 30-59 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	0
Commercial Portfolio Segment | Equipment lease financing | 60-89 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	0
Commercial Portfolio Segment | Equipment lease financing | 90 Days or More Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	0
Commercial Portfolio Segment | Equipment lease financing | Nonperforming Loans
Financing Receivable [Line Items]
Financing receivable, aging analysis	0
Consumer Portfolio Segment
Financing Receivable [Line Items]
Financing receivable, aging analysis	200	199
Consumer Portfolio Segment | Current or Less Than 30 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	58	59
Consumer Portfolio Segment | 30-59 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	4	4
Consumer Portfolio Segment | 60-89 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	4	5
Consumer Portfolio Segment | 90 Days or More Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	7	8
Consumer Portfolio Segment | Nonperforming Loans
Financing Receivable [Line Items]
Financing receivable, aging analysis	127	123
Consumer Portfolio Segment | Residential real estate
Financing Receivable [Line Items]
Financing receivable, aging analysis	103	102
Consumer Portfolio Segment | Residential real estate | Current or Less Than 30 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	7	8
Consumer Portfolio Segment | Residential real estate | 30-59 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	0	0
Consumer Portfolio Segment | Residential real estate | 60-89 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	0	1
Consumer Portfolio Segment | Residential real estate | 90 Days or More Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	0	1
Consumer Portfolio Segment | Residential real estate | Nonperforming Loans
Financing Receivable [Line Items]
Financing receivable, aging analysis	96	92
Consumer Portfolio Segment | Home equity
Financing Receivable [Line Items]
Financing receivable, aging analysis	33	31
Consumer Portfolio Segment | Home equity | Current or Less Than 30 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	4	2
Consumer Portfolio Segment | Home equity | 30-59 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	0	0
Consumer Portfolio Segment | Home equity | 60-89 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	0	0
Consumer Portfolio Segment | Home equity | 90 Days or More Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	0	0
Consumer Portfolio Segment | Home equity | Nonperforming Loans
Financing Receivable [Line Items]
Financing receivable, aging analysis	29	29
Consumer Portfolio Segment | Credit card
Financing Receivable [Line Items]
Financing receivable, aging analysis	57	60
Consumer Portfolio Segment | Credit card | Current or Less Than 30 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	41	44
Consumer Portfolio Segment | Credit card | 30-59 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	4	4
Consumer Portfolio Segment | Credit card | 60-89 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	4	4
Consumer Portfolio Segment | Credit card | 90 Days or More Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	7	7
Consumer Portfolio Segment | Credit card | Nonperforming Loans
Financing Receivable [Line Items]
Financing receivable, aging analysis	1	1
Consumer Portfolio Segment | Education
Financing Receivable [Line Items]
Financing receivable, aging analysis	4	5
Consumer Portfolio Segment | Education | Current or Less Than 30 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	4	5
Consumer Portfolio Segment | Education | 30-59 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	0	0
Consumer Portfolio Segment | Education | 60-89 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	0	0
Consumer Portfolio Segment | Education | 90 Days or More Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	0	0
Consumer Portfolio Segment | Education | Nonperforming Loans
Financing Receivable [Line Items]
Financing receivable, aging analysis	0	0
Consumer Portfolio Segment | Other consumer
Financing Receivable [Line Items]
Financing receivable, aging analysis	3	1
Consumer Portfolio Segment | Other consumer | Current or Less Than 30 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	2	0
Consumer Portfolio Segment | Other consumer | 30-59 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	0	0
Consumer Portfolio Segment | Other consumer | 60-89 Days Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	0	0
Consumer Portfolio Segment | Other consumer | 90 Days or More Past Due
Financing Receivable [Line Items]
Financing receivable, aging analysis	0	0
Consumer Portfolio Segment | Other consumer | Nonperforming Loans
Financing Receivable [Line Items]
Financing receivable, aging analysis	$ 1	$ 1