Credit Quality and the Allowance for Loan and Lease Losses - Summary of Loans That Were Modified for Borrowers Experiencing Financial Difficulty (Details) - USD ($) $ in Millions	Mar. 31, 2025	Mar. 31, 2024
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	$ 642	$ 592
Current
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	545	469
30-89 days past due
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	22	47
90 Days or More
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	75	76
Commercial | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	252	274
Commercial | Commercial and industrial loans | Current
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	213	207
Commercial | Commercial and industrial loans | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	6	11
Commercial | Commercial and industrial loans | 90 Days or More
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	33	56
Commercial | Commercial mortgage owner-occupied loans
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	90	37
Commercial | Commercial mortgage owner-occupied loans | Current
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	66	21
Commercial | Commercial mortgage owner-occupied loans | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	0	16
Commercial | Commercial mortgage owner-occupied loans | 90 Days or More
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	24	0
Commercial | Commercial mortgage nonowner-occupied loans
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	72	53
Commercial | Commercial mortgage nonowner-occupied loans | Current
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	72	50
Commercial | Commercial mortgage nonowner-occupied loans | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	0	3
Commercial | Commercial mortgage nonowner-occupied loans | 90 Days or More
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	0	0
Commercial | Commercial construction loans
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	108	74
Commercial | Commercial construction loans | Current
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	107	74
Commercial | Commercial construction loans | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	1	0
Commercial | Commercial construction loans | 90 Days or More
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	0	0
Residential Mortgage
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	81	110
Residential Mortgage | Current
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	55	82
Residential Mortgage | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	11	12
Residential Mortgage | 90 Days or More
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	15	16
Consumer | Home equity
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	15	15
Consumer | Home equity | Current
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	13	13
Consumer | Home equity | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	1	1
Consumer | Home equity | 90 Days or More
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	1	1
Consumer | Credit card
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	20	24
Consumer | Credit card | Current
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	15	17
Consumer | Credit card | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	3	4
Consumer | Credit card | 90 Days or More
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	2	3
Consumer | Solar energy installation loans
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	1	1
Consumer | Solar energy installation loans | Current
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	1	1
Consumer | Solar energy installation loans | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	0	0
Consumer | Solar energy installation loans | 90 Days or More
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	0	0
Consumer | Other consumer loans
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	3	4
Consumer | Other consumer loans | Current
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	3	4
Consumer | Other consumer loans | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	0	0
Consumer | Other consumer loans | 90 Days or More
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	$ 0	$ 0