MORTGAGE BANKING - Economic Assumptions Used to Estimate Value of MSRs Capitalized (Details) - Residential mortgages	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Servicing Assets at Fair Value [Line Items]
Weighted-average life (in years)	7 years 3 months 18 days	6 years 1 month 6 days	5 years 10 months 24 days
Weighted-average constant prepayment rate	8.70%	11.00%	10.70%
Weighted-average discount rate	9.80%	9.70%	9.70%