MORTGAGE BANKING - Residential Mortgage Loans (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Residential mortgage loan sale proceeds		$ 3,161	$ 2,652	$ 2,381
Valuation recoveries		2	4	9
Residential mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Residential mortgage loan sale proceeds	[1]	3,161	2,652	2,667
Gain on sales		35	69	51
Mortgage servicing fees		53	51	55
Repurchased residential mortgages		3	6	10
Valuation recoveries		$ (2)	$ (4)	$ (9)

[1] Represents the unpaid principal balance at the time of the sale.