LOANS AND LEASES - Purchases and Sales (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Purchases	$ 200	$ 448
Sales	0	0
Retail | Education
Financing Receivable, Allowance for Credit Losses [Line Items]
Purchases	0	325
Sales	0	0
Retail | Automobile
Financing Receivable, Allowance for Credit Losses [Line Items]
Purchases	0	123
Sales	0	0
Retail | Residential mortgages
Financing Receivable, Allowance for Credit Losses [Line Items]
Purchases	0	0
Sales	0	0
Retail | Home equity loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Purchases	0	0
Sales	0	0
Commercial | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Purchases	200	0
Sales	$ 0	$ 0