LOANS AND LEASES - Summary of Loans and Leases Portfolio (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 127,528	$ 119,088
Loans receivable held for sale	3,300	3,300
SBA Loans Serviced For Others | CBNA and Other Subsidiaries
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	43	33
Mortgage loans serviced for others | CBNA and Other Subsidiaries
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	79,200	77,500
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	66,032	57,538
Commercial | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	49,092	41,479
Commercial | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	14,502	13,522
Commercial | Leases
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	2,438	2,537
Retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	61,496	61,550
Retail | Residential mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	18,721	19,083
Retail | Home equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	12,992	13,154
Retail | Automobile
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	12,157	12,120
Retail | Education
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	10,887	10,347
Retail | Other retail
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and leases	$ 6,739	$ 6,846