ACQUISITIONS - Narrative (Details) $ / shares in Units, $ in Millions		3 Months Ended	9 Months Ended	12 Months Ended
	Apr. 06, 2022 USD ($) branch $ / shares shares	Mar. 31, 2021 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Feb. 18, 2022 USD ($) branch
Business Acquisition [Line Items]
Goodwill			$ 8,173	$ 8,188	$ 8,173	$ 7,116
Credit mark on PCD loans	$ 101
Income before income tax expense				2,030	2,655	2,977
Cumulative costs incurred in connection with acquisition		$ 13
Allowance on PCD loans and leases at acquisition					101
Provision expense (benefit) for loans and leases				$ 724	$ 394	$ (360)
Financial Asset Acquired and No Credit Deterioration
Business Acquisition [Line Items]
Provision expense (benefit) for loans and leases			$ 145
Online Deposit Business And 80 East Coast Branches Of HSBC Bank
Business Acquisition [Line Items]
Deposits							$ 6,300
Loans receivable, net							1,500
Goodwill							$ 120
Online Deposit Business And 80 East Coast Branches Of HSBC Bank | New York City Metro
Business Acquisition [Line Items]
Number of branches acquired | branch							66
Online Deposit Business And 80 East Coast Branches Of HSBC Bank | Mid-Atlantic/Washington D.C.
Business Acquisition [Line Items]
Number of branches acquired | branch							9
Online Deposit Business And 80 East Coast Branches Of HSBC Bank | Southeast Florida
Business Acquisition [Line Items]
Number of branches acquired | branch							5
Investors Acquisition
Business Acquisition [Line Items]
Number of branches acquired | branch	154
Goodwill	$ 887
Number of shares issued for acquisition | shares	73,600,000
Cash consideration, in dollars per share for each share of investors stock (in dollars per share) | $ / shares	$ 1.46
Stock options granted (in shares) | shares	1,151,301
Accretable discount on PCD loans	$ 320
Accretable discount on PCD loans, interest rate mark	179
Accretable discount on non-PCD loans, credit mark	141
Intangible asset, amortization period				10 years
Allowance on PCD loans and leases at acquisition	101
Fair value of acquired receivables	20,139
Unpaid principal balance of acquired receivables	20,560
Investors Acquisition | Acquisition-related Costs
Business Acquisition [Line Items]
Income before income tax expense				$ 335
Provision expense (benefit) for loans and leases				$ 145
Investors Acquisition | Financial Asset Acquired and No Credit Deterioration
Business Acquisition [Line Items]
Fair value of acquired receivables	15,600
Unpaid principal balance of acquired receivables	15,900
Investors Acquisition | Financial Asset Acquired with Credit Deterioration
Business Acquisition [Line Items]
Fair value of acquired receivables	4,500
Unpaid principal balance of acquired receivables	$ 4,700
Investors Acquisition | Restricted Stock
Business Acquisition [Line Items]
Awards granted (in shares) | shares	259,316
Investors Acquisition | Common Stock
Business Acquisition [Line Items]
Stock consideration (in dollars per each share) | $ / shares	$ 0.297