MORTGAGE BANKING - Changes Related to MSRs (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Valuation allowance for servicing assets:
Valuation charge-offs (recoveries)			$ (1)	$ 6
Residential mortgages
MSRs:
Balance as of beginning of period	$ 170	$ 166	167	173
Amount capitalized	9	10	28	20
Amortization	(8)	(9)	(24)	(26)
Carrying amount before valuation allowance	171	167	171	167
Valuation allowance for servicing assets:
Balance at beginning of period	4	13	5	9
Valuation charge-offs (recoveries)	0	2	(1)	6
Balance at end of period	4	15	4	15
Net carrying value of MSRs	$ 167	$ 152	$ 167	$ 152