LOANS RECEIVABLE AND ALLOWANCE FOR LOAN LOSSES - Allowance for Credit Losses (Details)	9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($) borrower	Sep. 30, 2020 USD ($) borrower	Dec. 31, 2020 USD ($)
Financing Receivable, Allowance For Credit Losses [Line Items]
Average loans	$ 239,492,000	$ 281,773,000
Net charge-offs to average loans (annualized)	(0.19%)	0.07%
Number of borrowers | borrower	20	30
Loans charged off	$ 193,000	$ 277,000	$ 392,000
Outstanding commitments	30,600,000	33,100,000
Unfunded commitments
Beginning balance	33,000	37,000	37,000
Reduction of unfunded reserve	(34,000)
Ending balance	456,000	29,000	$ 33,000
ASU 2016-13
Unfunded commitments
Ending balance	$ 457,000	$ (8,000)