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LOANS - Additional information (Details)		3 Months Ended	12 Months Ended
LOANS - Additional information (Details)	Mar. 27, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($) loan	Dec. 31, 2019 USD ($) item	Apr. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)
LOANS
Unamortized Loan Commitment and Origination Fees and Unamortized Discounts or Premiums, Total			$ 6,286,000	$ 2,482,000
Recorded investment - with allowance			8,082,000	3,375,000
Specific allowances			925,000	$ 1,051,000
Number of commercial relationship of TDRs that defaulted \| item				1
Provision for loan losses			3,913,000	$ 849,000
Loans receivable			1,644,209,000	1,182,222,000
Increase in expenses				3,064,000
Allowance for Loan losses			$ 11,385,000	$ 9,836,000		$ 9,309,000
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment			1	11
Financing receivable charge-offs			$ 2,465,000	$ 379,000
Recorded Investment			17,818,000	5,486,000
Financing Receivable, Troubled Debt Restructuring, Commitment to Lend			0	0
Loans Modified, Recorded Investment			37,397,000
Financial Assets Facing Additional Credit Deterioration Due to Economic Impact of Covid-19
LOANS
Provision for loan losses			785,000
Commercial
LOANS
Loans receivable			1,007,751,000	578,901,000
Net decrease in allowance			1,675,000
Allowance for Loan losses			6,308,000	4,788,000		4,591,000
Financing receivable charge-offs			2,343,000	6,000
Loans Modified, Recorded Investment			35,002,000
Commercial \| Maximum
LOANS
Threshold amount of loan balance for evaluating individual impairment loss			200,000
Commercial loan of $3,500,000
LOANS
Provision for loan losses			3,500,000
Specific Loans Portfolio Segment
LOANS
Provision for loan losses			2,238,000
Unallocated
LOANS
Allowance for Loan losses			585,000	585,000		499,000
Financing receivable charge-offs			0	0
Residential mortgage
LOANS
Loans receivable			620,172,000	586,580,000
Allowance for Loan losses			4,253,000	4,182,000		3,986,000
Financing receivable charge-offs			0	190,000
Loans Modified, Recorded Investment			2,334,000
Residential mortgage \| Maximum
LOANS
Threshold amount of loan balance for evaluating individual impairment loss			400,000
Small Business Administration - Paycheck Protection Program
LOANS
Unamortized Loan Commitment and Origination Fees and Unamortized Discounts or Premiums, Total			3,037,000
Amount of stimulus package designed	$ 2,000,000,000,000
Contractual principal balances			134,802,000
Accretion of fees received			1,945,000
Recorded Investment			132,269,000
Adjustment recorded in acquisition			504,000
Small Business Administration - Paycheck Protection Program \| Commercial
LOANS
Loans receivable			132,269,000	0
Allowance for Loan losses			0
Construction Loans \| Commercial
LOANS
Recorded investment - with allowance			0	1,261,000
Specific allowances			0	678,000
Loans receivable			42,874,000	33,555,000
Allowance for Loan losses			454,000	966,000		110,000
Financing receivable charge-offs			107,000	0
Construction Loans \| Residential mortgage
LOANS
Loans receivable			20,613,000	14,798,000
Allowance for Loan losses			99,000	117,000		203,000
Financing receivable charge-offs			0	0
Real estate loan
LOANS
Specific allowances			416,000	678,000
Provision for loan losses		$ 107,000
Real estate loan \| Commercial
LOANS
Recorded investment - with allowance			6,501,000	0
Specific allowances			691,000	0
Provision for loan losses			2,364,000
Loans receivable			531,810,000	301,227,000
Allowance for Loan losses			$ 3,051,000	1,921,000		$ 2,538,000
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment \| loan			1
Financing receivable charge-offs			$ 0	0
Real estate loan \| Commercial \| TDRs for which payment defaults occurred
LOANS
Specific allowances			0	0
Real estate loan \| Commercial loan of $3,500,000
LOANS
Financing receivable charge-offs			2,219,000
Real estate loan \| Specific Loans Portfolio Segment
LOANS
Net decrease in allowance			126,000
Real estate loan \| Residential mortgage
LOANS
Specific allowances			$ 153,000	$ 176,000
Loans Receivable 1 \| Credit Availability Concentration Risk
LOANS
Concentration Risk, Percentage			10.00%	10.00%
Purchased Credit Impaired
LOANS
Loans receivable			$ 6,841,000	$ 441,000
Loans Modified, Recorded Investment			6,841,000	441,000
Purchased Credit Impaired \| Commercial
LOANS
Loans receivable			6,696,000	364,000
Purchased Credit Impaired \| Residential mortgage
LOANS
Loans receivable			145,000	77,000
Purchased Credit Impaired \| Small Business Administration - Paycheck Protection Program \| Commercial
LOANS
Loans receivable			0
Purchased Credit Impaired \| Construction Loans \| Commercial
LOANS
Loans receivable			0	0
Purchased Credit Impaired \| Construction Loans \| Residential mortgage
LOANS
Loans receivable			0	0
Purchased Credit Impaired \| Real estate loan \| Commercial
LOANS
Loans receivable			4,298,000	364,000
Market Rate Adjustment
LOANS
Adjustment recorded in acquisition			$ 2,909,000	$ (1,807,000)
Monument Bancorp, Inc
LOANS
Loans					$ 259,300,000