LOANS AND LEASES - Loan Purchases and Sales (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses [Line Items]
Purchases	$ 1,015	$ 2,458
Sales	886	901
Retail | Education
Financing Receivable, Allowance for Credit Losses [Line Items]
Purchases	862	1,224
Sales	0	0
Retail | Automobile
Financing Receivable, Allowance for Credit Losses [Line Items]
Purchases	153	695
Sales	0	0
Retail | Residential mortgages
Financing Receivable, Allowance for Credit Losses [Line Items]
Purchases	0	539
Sales	254	699
Retail | Home equity loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Purchases	0	0
Sales	29	55
Commercial | Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Purchases	0	0
Sales	$ 603	$ 147