LOANS AND LEASES - Schedule of Nonaccrual Status (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual Loans	$ 1,074	$ 969
Loans > 90 Days and Accruing	158	123
Financing receivable, accrued interest reversed to nonaccrual status	3	10
Financing receivable, nonaccrual without allowance for credit loss	195	138
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual Loans	805	698
Loans > 90 Days and Accruing	144	114
Commercial | Commercial construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual Loans	1	2
Loans > 90 Days and Accruing	0	1
Commercial | Owner occupied commercial mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual Loans	65	60
Loans > 90 Days and Accruing	10	8
Commercial | Non-owner occupied commercial mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual Loans	434	411
Loans > 90 Days and Accruing	124	39
Commercial | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual Loans	275	194
Loans > 90 Days and Accruing	9	59
Commercial | Leases
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual Loans	30	31
Loans > 90 Days and Accruing	1	7
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual Loans	145	154
Loans > 90 Days and Accruing	14	7
Consumer | Residential mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual Loans	118	127
Loans > 90 Days and Accruing	11	4
Consumer | Revolving mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual Loans	21	21
Loans > 90 Days and Accruing	0	0
Consumer | Consumer auto
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual Loans	5	5
Loans > 90 Days and Accruing	0	0
Consumer | Consumer other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual Loans	1	1
Loans > 90 Days and Accruing	3	3
SVB
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual Loans	124	117
Loans > 90 Days and Accruing	0	2
SVB | Global fund banking
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual Loans	0	0
Loans > 90 Days and Accruing	0	0
SVB | Investor dependent - early stage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual Loans	51	37
Loans > 90 Days and Accruing	0	2
SVB | Investor dependent - growth stage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual Loans	37	37
Loans > 90 Days and Accruing	0	0
SVB | Innovation C&I and cash flow dependent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual Loans	36	43
Loans > 90 Days and Accruing	$ 0	$ 0