FAIR VALUE OF ASSETS AND LIABILITIES - Balance Sheet Grouping (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Financial assets:
Cash and cash equivalents	$ 248,972		$ 211,616
Securities available for sale, at fair value	262,710		239,473
Securities held to maturity, at amortized cost			26,372
Federal Home Loan Bank stock	15,786		17,121
Loans held for sale	158,898		110,552
Loans, net	3,437,915		3,147,498
Retirement plan annuities	13,537		13,333
Mortgage servicing rights	16,127	$ 13,207	17,150	$ 18,156	$ 20,231	$ 22,217
Accrued interest receivable	11,979		9,807
Financial liabilities:
Deposits	3,308,674		2,942,873
Subordinated debt	33,970		33,907
Mortgagors' escrow accounts	8,299		6,053
Accrued interest payable	1,451		1,669
Carrying Amount
Financial assets:
Cash and cash equivalents	248,972		211,616
Securities available for sale, at fair value	262,710		239,473
Securities held to maturity, at amortized cost			26,372
Federal Home Loan Bank stock	15,786		17,121
Loans held for sale	158,898		110,552
Loans, net	3,437,915		3,147,498
Retirement plan annuities	13,537		13,333
Accrued interest receivable	11,979		9,807
Financial liabilities:
Deposits	3,308,674		2,942,873
Borrowed funds	341,114		354,132
Subordinated debt	33,970		33,907
Mortgagors' escrow accounts	8,299		6,053
Accrued interest payable	1,451		1,669
Carrying Amount | Derivative loan commitments
Derivative commitments/agreements:
Assets	10,822		1,385
Liabilities	83		174
Carrying Amount | Interest rate management agreement
Derivative commitments/agreements:
Liabilities	1,762
Carrying Amount | Interest rate swaps
Derivative commitments/agreements:
Assets	46,855		15,092
Liabilities	46,855		15,092
Carrying Amount | Forward loan sale commitments
Derivative commitments/agreements:
Assets	22		26
Liabilities	1,984		158
Fair Value
Financial assets:
Cash and cash equivalents	248,972		211,616
Securities available for sale, at fair value	262,710		239,473
Securities held to maturity, at amortized cost			26,927
Loans held for sale	158,898		110,552
Loans, net	3,479,805		3,176,442
Retirement plan annuities	13,537		13,333
Accrued interest receivable	11,979		9,807
Financial liabilities:
Deposits	3,316,033		2,943,899
Borrowed funds	344,825		354,881
Subordinated debt	35,481		34,619
Mortgagors' escrow accounts	8,299		6,053
Accrued interest payable	1,451		1,669
Fair Value | Derivative loan commitments
Derivative commitments/agreements:
Assets	10,822		1,385
Liabilities	83		174
Fair Value | Interest rate management agreement
Derivative commitments/agreements:
Liabilities	1,762
Fair Value | Interest rate swaps
Derivative commitments/agreements:
Assets	46,855		15,092
Liabilities	46,855		15,092
Fair Value | Forward loan sale commitments
Derivative commitments/agreements:
Assets	22		26
Liabilities	1,984		158
Fair Value | Level 1
Financial assets:
Cash and cash equivalents	248,972		211,616
Fair Value | Level 2
Financial assets:
Securities available for sale, at fair value	262,710		239,473
Securities held to maturity, at amortized cost			26,927
Loans held for sale	158,898		110,552
Accrued interest receivable	11,979		9,807
Financial liabilities:
Borrowed funds	344,825		354,881
Accrued interest payable	1,451		1,669
Fair Value | Level 2 | Interest rate management agreement
Derivative commitments/agreements:
Liabilities	1,762
Fair Value | Level 2 | Interest rate swaps
Derivative commitments/agreements:
Assets	46,855		15,092
Liabilities	46,855		15,092
Fair Value | Level 3
Financial assets:
Loans, net	3,479,805		3,176,442
Retirement plan annuities	13,537		13,333
Financial liabilities:
Deposits	3,316,033		2,943,899
Subordinated debt	35,481		34,619
Mortgagors' escrow accounts	8,299		6,053
Fair Value | Level 3 | Derivative loan commitments
Derivative commitments/agreements:
Assets	10,822		1,385
Liabilities	83		174
Fair Value | Level 3 | Forward loan sale commitments
Derivative commitments/agreements:
Assets	22		26
Liabilities	$ 1,984		$ 158