FAIR VALUE MEASUREMENTS - Information about Significant Unobservable Inputs Used for Recurring Fair Value Measurements for Level 3 MSRs (Details) - Fair Value, Measurements, Recurring - Level 3 - Mortgage Servicing Rights - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Unobservable input, discount rate	10.00%	10.00%
Unobservable input, prepayment rate	4.50%	4.60%
Unobservable input, delinquency rate	0.00%	0.00%
Unobservable input, cost to service	$ 88	$ 84
Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Unobservable input, discount rate	15.00%	15.00%
Unobservable input, prepayment rate	13.30%	22.30%
Unobservable input, delinquency rate	6.00%	13.00%
Unobservable input, cost to service	$ 135	$ 181
Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Unobservable input, discount rate	10.40%	10.40%
Unobservable input, prepayment rate	7.60%	9.40%
Unobservable input, delinquency rate	2.30%	2.20%
Unobservable input, cost to service	$ 107	$ 102