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 ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Unobservable input, Discount rate	9.00%	10.00%
Unobservable input, Prepayment rate	4.50%	4.60%
Unobservable input, Delinquency rate	0.00%	0.00%
Unobservable input, Cost to service	$ 82	$ 84
Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Unobservable input, Discount rate	12.00%	15.00%
Unobservable input, Prepayment rate	11.30%	22.30%
Unobservable input, Delinquency rate	5.00%	13.00%
Unobservable input, Cost to service	$ 132	$ 181
Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Unobservable input, Discount rate	9.40%	10.40%
Unobservable input, Prepayment rate	7.10%	9.40%
Unobservable input, Delinquency rate	2.30%	2.20%
Unobservable input, Cost to service	$ 105	$ 102