MORTGAGE BANKING - Changes Related to MSRs (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Valuation allowance for servicing assets:
Valuation recoveries	$ (3)	$ 0
Residential mortgages
MSRs:
Balance as of beginning of period	201	168
Amount capitalized	8	10
Amortization	(8)	(8)
Carrying amount before valuation allowance	201	170
Valuation allowance for servicing assets:
Balance at beginning of period	3	5
Valuation recoveries	(3)	0
Balance at end of period	0	5
Net carrying value of MSRs	$ 201	$ 165