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	Sep. 30, 2025	Sep. 30, 2024
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	$ 716	$ 453
Current
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	619	394
30-89 days past due
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	63	32
90 Days or More
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	34	27
Commercial | Commercial and industrial loans
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	246	219
Commercial | Commercial and industrial loans | Current
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	226	196
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	2	13
Commercial | Commercial and industrial loans | 90 Days or More
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	18	10
Commercial | Commercial mortgage owner-occupied loans
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	112	26
Commercial | Commercial mortgage owner-occupied loans | Current
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	112	26
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	0
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	0	0
Commercial | Commercial mortgage nonowner-occupied loans
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	117	67
Commercial | Commercial mortgage nonowner-occupied loans | Current
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	117	67
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	0
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	113	6
Commercial | Commercial construction loans | Current
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	69	6
Commercial | Commercial construction loans | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	44	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	88	94
Residential Mortgage | Current
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	62	66
Residential Mortgage | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	13	15
Residential Mortgage | 90 Days or More
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	13	13
Consumer | Home equity
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	16	15
Consumer | Home equity | Current
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	14	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	18	22
Consumer | Credit card | Current
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	13	16
Consumer | Credit card | 30-89 days past due
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	3	3
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	2	1
Consumer | Solar energy installation loans | Current
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	2	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	4	3
Consumer | Other consumer loans | Current
Financing Receivable, Recorded Investment, Past Due
Financing receivable, excluding accrued interest, modified, after 12 months	4	3
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