LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES - Allowance for credit losses, average loans (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
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

Amount of direct write-downs of financing receivables charged against the allowance.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration