Asset Quality - Schedule of Performance of Loans That Have Been Modified in the Last 12 Months (Details) - USD ($) $ in Millions	Mar. 31, 2024	Mar. 31, 2023
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	$ 355	$ 93
Current
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	259	89
30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	41	3
90 and Greater Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	55	1
Total commercial loans
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	324	88
Total commercial loans | Current
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	232	84
Total commercial loans | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	38	3
Total commercial loans | 90 and Greater Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	54	1
Total commercial loans | Commercial and Industrial
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	254	84
Total commercial loans | Commercial and Industrial | Current
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	184	80
Total commercial loans | Commercial and Industrial | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	16	3
Total commercial loans | Commercial and Industrial | 90 and Greater Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	54	1
Total commercial loans | Total commercial real estate loans
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	324	88
Total commercial loans | Total commercial real estate loans | Current
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	232	84
Total commercial loans | Total commercial real estate loans | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	38	3
Total commercial loans | Total commercial real estate loans | 90 and Greater Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	54	1
Total commercial loans | Commercial mortgage
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	51	4
Total commercial loans | Commercial mortgage | Current
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	29	4
Total commercial loans | Commercial mortgage | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	22	0
Total commercial loans | Commercial mortgage | 90 and Greater Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	0	0
Total commercial loans | Construction
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	19	0
Total commercial loans | Construction | Current
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	19	0
Total commercial loans | Construction | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	0	0
Total commercial loans | Construction | 90 and Greater Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	0	0
Total commercial loans | Commercial lease financing
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	0	0
Total commercial loans | Commercial lease financing | Current
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	0	0
Total commercial loans | Commercial lease financing | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	0	0
Total commercial loans | Commercial lease financing | 90 and Greater Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	0	0
Total consumer loans
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	31	5
Total consumer loans | Current
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	27	5
Total consumer loans | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	3	0
Total consumer loans | 90 and Greater Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	1	0
Total consumer loans | Real estate — residential mortgage
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	13	1
Total consumer loans | Real estate — residential mortgage | Current
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	11	1
Total consumer loans | Real estate — residential mortgage | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	2	0
Total consumer loans | Real estate — residential mortgage | 90 and Greater Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	0	0
Total consumer loans | Home equity loans
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	11	2
Total consumer loans | Home equity loans | Current
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	9	2
Total consumer loans | Home equity loans | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	1	0
Total consumer loans | Home equity loans | 90 and Greater Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	1	0
Total consumer loans | Other consumer loans
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	3	1
Total consumer loans | Other consumer loans | Current
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	3	1
Total consumer loans | Other consumer loans | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	0	0
Total consumer loans | Other consumer loans | 90 and Greater Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	0	0
Total consumer loans | Credit cards
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	4	1
Total consumer loans | Credit cards | Current
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	4	1
Total consumer loans | Credit cards | 30-89 Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	0	0
Total consumer loans | Credit cards | 90 and Greater Days Past Due
Financing Receivable, Past Due [Line Items]
Total loans modified in last 12 months	$ 0	$ 0