ACQUISITIONS - Narrative (Details) $ / shares in Units, $ in Millions		3 Months Ended				6 Months Ended
	Apr. 06, 2022 USD ($) branch $ / shares shares	Jun. 30, 2022 USD ($)	Jun. 30, 2022 USD ($) shares	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Jun. 30, 2022 USD ($) segment	Jun. 30, 2021 USD ($)	Feb. 18, 2022 USD ($) branch	Dec. 31, 2021 USD ($)
Business Acquisition [Line Items]
Goodwill		$ 8,081	$ 8,081			$ 8,081			$ 7,116
Cumulative costs incurred in connection with acquisition					$ 13
Number of operating segments | segment						2
Credit mark on PCD loans	$ 101
Income before income tax expense			478	$ 831		$ 1,014	$ 1,612
Allowance on PCD loans and leases at acquisition			101			101
Provision expense (benefit) for loans and leases			$ 192	$ (169)		$ 213	$ (260)
Financial Asset Acquired and No Credit Deterioration
Business Acquisition [Line Items]
Provision expense (benefit) for loans and leases		$ 145
80 Branches and Deposit Business, HSBC Bank
Business Acquisition [Line Items]
Deposits								$ 6,300
Loans receivable, net								1,500
Goodwill								$ 119
80 Branches and Deposit Business, HSBC Bank | New York City Metro
Business Acquisition [Line Items]
Number of branches acquired | branch								66
80 Branches and Deposit Business, HSBC Bank | Mid-Atlantic/Washington D.C.
Business Acquisition [Line Items]
Number of branches acquired | branch								9
80 Branches and Deposit Business, 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	$ 799
Number of shares issued for acquisition | shares	73,600,000
Dilutive effect of acquisition (in shares) | shares			68,600,000
Cash consideration, in dollars per share for each share of investors stock (in dollars per share) | $ / shares	$ 1.46
Options granted (in shares) | shares	1,151,301
Accretable discount on PCD loans	$ 300
Accretable discount on PCD loans, interest rate mark	159
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	20,158
Unpaid principal balance of acquired receivables	20,560
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 | Financial Asset Acquired and No Credit Deterioration
Business Acquisition [Line Items]
Fair value of acquired receivables	15,700
Unpaid principal balance of acquired receivables	$ 15,900
Investors Acquisition | Restricted Stock
Business Acquisition [Line Items]
Share-based payment 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