ALLOWANCE FOR CREDIT LOSSES, NONACCRUAL LOANS AND LEASES, AND CONCENTRATIONS OF CREDIT RISK - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021
Financing Receivable, Troubled Debt Restructuring [Line Items]
Charge-offs	$ 59
Provision expense (benefit) for loans and leases	3	$ (140)
Allowance for credit loss, reserve build	$ 56
Economic forecast that generally reflects real GDP growth on an annual average basis	2.50%		2.80%
Economic forecast that generally reflects average unemployment rate	5.20%		6.00%
Loans and leases	$ 131,305		$ 128,163
Mortgage loans collateralized by OREO	189		142
Troubled debt restructuring, charge-offs	1	$ 2
TDR commitment to lend	76		56
80 Branches And Deposit Business, HSBC Bank
Financing Receivable, Troubled Debt Restructuring [Line Items]
Provision expense (benefit) for loans and leases	24
Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Charge-offs decrease	93
Loans and leases	61,521		60,350
Retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Charge-offs decrease	6
Loans and leases	69,784		67,813
Commercial and industrial | Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans and leases	45,724		44,500
Collateral Dependent | Home Equity Loan and Residential Mortgages | Retail
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans and leases	579		542
Collateral Dependent | Commercial and industrial | Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Loans and leases	$ 21		$ 103