MORTGAGE SERVICING RIGHTS - Balance of loans serviced for others and fair value of mortgage servicing rights (Details) - Balance of loans serviced for others and fair value of mortgage servicing rights - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Residential mortgage loans serviced for others	$ 116,096	$ 117,538
Fair value of mortgage servicing rights	$ 1,048	$ 1,010