LOANS AND LEASES - Schedule of Amortized Cost of Loans Modified (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 202	$ 92	$ 696
Percent of Total Loan Class	0.15%	0.07%
Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 104	$ 83
Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	30	0
Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1	0
Term Extension and Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	12	6
Term Extension and Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	55
Other Combinations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	3
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 125	$ 85	501
Percent of Total Loan Class	0.19%	0.16%
Commercial | Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 79	$ 76
Commercial | Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
Commercial | Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1	0
Commercial | Term Extension and Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	9	6
Commercial | Term Extension and Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	36
Commercial | Other Combinations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	3
Commercial | Commercial construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 3	$ 1	4
Percent of Total Loan Class	0.08%	0.02%
Commercial | Commercial construction | Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 3	$ 1
Commercial | Commercial construction | Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
Commercial | Commercial construction | Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
Commercial | Commercial construction | Term Extension and Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
Commercial | Commercial construction | Term Extension and Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
Commercial | Commercial construction | Other Combinations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
Commercial | Owner occupied commercial mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 26	$ 10	29
Percent of Total Loan Class	0.16%	0.06%
Commercial | Owner occupied commercial mortgage | Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 15	$ 10
Commercial | Owner occupied commercial mortgage | Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
Commercial | Owner occupied commercial mortgage | Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1	0
Commercial | Owner occupied commercial mortgage | Term Extension and Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1	0
Commercial | Owner occupied commercial mortgage | Term Extension and Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	9
Commercial | Owner occupied commercial mortgage | Other Combinations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
Commercial | Non-owner occupied commercial mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 65	$ 53	353
Percent of Total Loan Class	0.42%	0.51%
Commercial | Non-owner occupied commercial mortgage | Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 38	$ 53
Commercial | Non-owner occupied commercial mortgage | Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
Commercial | Non-owner occupied commercial mortgage | Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
Commercial | Non-owner occupied commercial mortgage | Term Extension and Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
Commercial | Non-owner occupied commercial mortgage | Term Extension and Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	27
Commercial | Non-owner occupied commercial mortgage | Other Combinations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
Commercial | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 31	$ 21	115
Percent of Total Loan Class	0.11%	0.09%
Commercial | Commercial and industrial | Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 23	$ 12
Commercial | Commercial and industrial | Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
Commercial | Commercial and industrial | Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
Commercial | Commercial and industrial | Term Extension and Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	8	6
Commercial | Commercial and industrial | Term Extension and Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
Commercial | Commercial and industrial | Other Combinations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	3
Commercial | Leases
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 0
Percent of Total Loan Class	0.00%
Commercial | Leases | Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 0
Commercial | Leases | Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
Commercial | Leases | Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
Commercial | Leases | Term Extension and Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
Commercial | Leases | Term Extension and Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
Commercial | Leases | Other Combinations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 10	$ 1	21
Percent of Total Loan Class	0.03%	0.01%
Consumer | Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 7	$ 1
Consumer | Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
Consumer | Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
Consumer | Term Extension and Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	3	0
Consumer | Term Extension and Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
Consumer | Other Combinations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
Consumer | Residential mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 8	$ 1	17
Percent of Total Loan Class	0.03%	0.01%
Consumer | Residential mortgage | Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 6	$ 1
Consumer | Residential mortgage | Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
Consumer | Residential mortgage | Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
Consumer | Residential mortgage | Term Extension and Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	2	0
Consumer | Residential mortgage | Term Extension and Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
Consumer | Residential mortgage | Other Combinations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
Consumer | Revolving mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 2		4
Percent of Total Loan Class	0.08%
Consumer | Revolving mortgage | Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 1
Consumer | Revolving mortgage | Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
Consumer | Revolving mortgage | Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
Consumer | Revolving mortgage | Term Extension and Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1
Consumer | Revolving mortgage | Term Extension and Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
Consumer | Revolving mortgage | Other Combinations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
Consumer | Consumer auto
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 0
Percent of Total Loan Class	0.00%
Consumer | Consumer auto | Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 0
Consumer | Consumer auto | Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
Consumer | Consumer auto | Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
Consumer | Consumer auto | Term Extension and Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
Consumer | Consumer auto | Term Extension and Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
Consumer | Consumer auto | Other Combinations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
Consumer | Consumer other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 0
Percent of Total Loan Class	0.00%
Consumer | Consumer other | Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 0
Consumer | Consumer other | Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
Consumer | Consumer other | Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
Consumer | Consumer other | Term Extension and Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
Consumer | Consumer other | Term Extension and Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
Consumer | Consumer other | Other Combinations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
SVB
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 67	$ 6	174
Percent of Total Loan Class	0.17%	0.01%
SVB | Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 18	$ 6
SVB | Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	30	0
SVB | Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
SVB | Term Extension and Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
SVB | Term Extension and Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	19
SVB | Other Combinations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
SVB | Global fund banking
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 0
Percent of Total Loan Class	0.00%
SVB | Global fund banking | Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 0
SVB | Global fund banking | Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
SVB | Global fund banking | Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
SVB | Global fund banking | Term Extension and Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
SVB | Global fund banking | Term Extension and Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
SVB | Global fund banking | Other Combinations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
SVB | Investor dependent - early stage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 8		27
Percent of Total Loan Class	0.63%
SVB | Investor dependent - early stage | Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 0
SVB | Investor dependent - early stage | Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	6
SVB | Investor dependent - early stage | Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
SVB | Investor dependent - early stage | Term Extension and Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
SVB | Investor dependent - early stage | Term Extension and Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	2
SVB | Investor dependent - early stage | Other Combinations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
SVB | Investor dependent - growth stage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 34	$ 6	63
Percent of Total Loan Class	1.25%	0.14%
SVB | Investor dependent - growth stage | Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 0	$ 6
SVB | Investor dependent - growth stage | Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	24	0
SVB | Investor dependent - growth stage | Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
SVB | Investor dependent - growth stage | Term Extension and Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
SVB | Investor dependent - growth stage | Term Extension and Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	10
SVB | Investor dependent - growth stage | Other Combinations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	$ 0
SVB | Innovation C&I and cash flow dependent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 25		$ 84
Percent of Total Loan Class	0.27%
SVB | Innovation C&I and cash flow dependent | Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 18
SVB | Innovation C&I and cash flow dependent | Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
SVB | Innovation C&I and cash flow dependent | Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
SVB | Innovation C&I and cash flow dependent | Term Extension and Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0
SVB | Innovation C&I and cash flow dependent | Term Extension and Payment Delay
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	7
SVB | Innovation C&I and cash flow dependent | Other Combinations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 0