FAIR VALUE OF ASSETS AND LIABILITIES - Balance Sheet Grouping (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Financial assets:
Cash and cash equivalents	$ 235,420	$ 211,616
Securities available for sale, at fair value	249,789	239,473
Securities held to maturity, at amortized cost		26,372
Federal Home Loan Bank stock	13,530	17,121
Loans held for sale	118,316	110,552
Loans, net	3,157,481	3,147,498
Retirement plan annuities	13,434	13,333
Mortgage servicing rights	13,207	17,150	$ 20,231	$ 22,217
Accrued interest receivable	9,710	9,807
Financial liabilities:
Deposits	3,021,224	2,942,873
Subordinated debt	33,938	33,907
Mortgagors' escrow accounts	8,279	6,053
Accrued interest payable	1,337	1,669
Carrying Amount
Financial assets:
Cash and cash equivalents	235,420	211,616
Securities available for sale, at fair value	249,789	239,473
Securities held to maturity, at amortized cost		26,372
Federal Home Loan Bank stock	13,530	17,121
Loans held for sale	118,316	110,552
Loans, net	3,157,481	3,147,498
Retirement plan annuities	13,434	13,333
Mortgage servicing rights	13,207	17,150
Accrued interest receivable	9,710	9,807
Financial liabilities:
Deposits	3,021,224	2,942,873
Borrowed funds	285,123	354,132
Subordinated debt	33,938	33,907
Mortgagors' escrow accounts	8,279	6,053
Accrued interest payable	1,337	1,669
Carrying Amount | Derivative loan commitments
Derivative commitments/agreements:
Assets	8,065	1,385
Liabilities	1,663	174
Carrying Amount | Interest rate swaps
Derivative commitments/agreements:
Assets	43,278	15,092
Liabilities	43,278	15,092
Carrying Amount | Forward loan sale commitments
Derivative commitments/agreements:
Assets	13	26
Liabilities	3,817	158
Fair Value
Financial assets:
Cash and cash equivalents	235,420	211,616
Securities available for sale, at fair value	249,789	239,473
Securities held to maturity, at amortized cost		26,927
Loans held for sale	118,316	110,552
Loans, net	3,212,565	3,176,442
Retirement plan annuities	13,434	13,333
Mortgage servicing rights	13,207	17,150
Accrued interest receivable	9,710	9,807
Financial liabilities:
Deposits	3,029,586	2,943,899
Borrowed funds	287,917	354,881
Subordinated debt	37,026	34,619
Mortgagors' escrow accounts	8,279	6,053
Accrued interest payable	1,337	1,669
Fair Value | Derivative loan commitments
Derivative commitments/agreements:
Assets	8,065	1,385
Liabilities	1,663	174
Fair Value | Interest rate swaps
Derivative commitments/agreements:
Assets	43,278	15,092
Liabilities	43,278	15,092
Fair Value | Forward loan sale commitments
Derivative commitments/agreements:
Assets	13	26
Liabilities	3,817	158
Fair Value | Level 1
Financial assets:
Cash and cash equivalents	235,420	211,616
Fair Value | Level 2
Financial assets:
Securities available for sale, at fair value	249,789	239,473
Securities held to maturity, at amortized cost		26,927
Loans held for sale	118,316	110,552
Mortgage servicing rights	13,207	17,150
Accrued interest receivable	9,710	9,807
Financial liabilities:
Borrowed funds	287,917	354,881
Accrued interest payable	1,337	1,669
Fair Value | Level 2 | Interest rate swaps
Derivative commitments/agreements:
Assets	43,278	15,092
Liabilities	43,278	15,092
Fair Value | Level 3
Financial assets:
Loans, net	3,212,565	3,176,442
Retirement plan annuities	13,434	13,333
Financial liabilities:
Deposits	3,029,586	2,943,899
Subordinated debt	37,026	34,619
Mortgagors' escrow accounts	8,279	6,053
Fair Value | Level 3 | Derivative loan commitments
Derivative commitments/agreements:
Assets	8,065	1,385
Liabilities	1,663	174
Fair Value | Level 3 | Forward loan sale commitments
Derivative commitments/agreements:
Assets	13	26
Liabilities	$ 3,817	$ 158