BUSINESS COMBINATIONS - Narrative (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions			3 Months Ended		12 Months Ended
	Apr. 04, 2023	Mar. 27, 2023	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Business Acquisition [Line Items]
Net interest income			$ 1,817	$ 850
Noninterest income			627	10,259
Net income			731	9,518
Operating Segments | Silicon Valley Bridge Bank Segment
Business Acquisition [Line Items]
Net interest income			546	37
Noninterest income			137	9
Net income			$ 202	18
Operating Segments | CIT Group Inc.
Business Acquisition [Line Items]
Net interest income				65
Noninterest income				14
Net income				$ 35
FDIC Credit Facility
Business Acquisition [Line Items]
FDIC advances, maximum amount available		$ 70,000
Purchase Money Note
Business Acquisition [Line Items]
Debt instrument, term		5 years
Principal amount		$ 36,070
Notes payable, fair value disclosure		35,810
SVB
Business Acquisition [Line Items]
Total assets acquired		107,539
Loans and leases, net of the initial PCD ALLL		68,468
Cash and interest-earning deposits at banks		35,310
Total liabilities assumed		61,423
Deposits		56,014
Assets acquired, discount		16,450
Business combination, financing receivable acquired, amount covered by share-loss agreement		$ 60,000
Business combination, financing receivable acquired, amount covered by share-loss agreement, coverage percentage for loss up to $5 billion		0.00%
Business combination, financing receivable acquired, amount covered by share-loss agreement, coverage percentage for loss in excess of $5 billion		$ 5,000
Business combination, financing receivable acquired, above threshold amount covered by share-loss agreement, percentage		50.00%
Share-Loss Agreement, reimbursement percentage		50.00%
Share-Loss Agreement, term		5 years
Share-Loss Agreement, reimbursement term		8 years
Loans funded after acquisition date, loss share term		1 year
Share-Loss Agreement, true-up payment term		45 days
True-up amount		$ 1,500
Cash settled value appreciation instrument issued and issuable (in shares)		5
Number of trading days		2 days
Cash settled value appreciation instrument, threshold amount (in dollars per share)		$ 582.55
Cash settled value appreciation instrument, maximum amount		$ 500
Payment to acquire business, value appreciation instrument exercised	$ 500
Gain on acquisition, after income taxes		9,808			$ 9,810
Deferred tax liabilities		3,364			$ 3,360
UPB		71,287
Purchase accounting adjustment of discount		2,300
SVB | Non-PCD
Business Acquisition [Line Items]
Loans and leases acquired, FV		66,420
UPB		$ 68,719
SVB | FDIC Credit Facility
Business Acquisition [Line Items]
FHLB, advances, interest rate		0.00%
SVB | Secured Debt | FDIC Credit Facility | Secured Overnight Financing Rate
Business Acquisition [Line Items]
Basis spread on variable rate		0.25%
SVB | Purchase Money Note
Business Acquisition [Line Items]
Debt instrument, term		5 years
Debt instrument, face amount		$ 35,000
SVB | Purchase Money Note | Secured Debt
Business Acquisition [Line Items]
Debt instrument, stated interest rate		3.50%