LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES - Allowance for credit losses (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) borrower	Mar. 31, 2017 USD ($) borrower	Dec. 31, 2017 USD ($)
LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES
Outstanding commitments	$ 24,700,000	$ 26,500,000
Beginning balance	24,000	25,000	$ 25,000
Reduction of unfunded reserve	(18,000)
Provisions charged to operations	6,000	17,000
Ending balance	12,000	42,000	24,000
Loans charged off	101,000	170,000	$ 566,000
Average loans	$ 273,964,000	$ 267,494,000
Net charge-offs to average loans (annualized)	0.07%	0.12%
Number of borrowers | borrower	7	11