FAIR VALUE OF ASSETS AND LIABILITIES - Balance Sheet Grouping (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Financial assets:
Cash and cash equivalents	$ 238,631		$ 105,521
Securities available for sale, at fair value	204,133		209,293
Securities held to maturity, at amortized cost	27,099		44,688
Federal Home Loan Bank stock	13,466		24,969
Loans held for sale	102,121		42,107
Loans, net	3,089,220		2,964,852
Retirement plan annuities	13,231		12,931
Mortgage servicing rights	16,067	$ 18,156	22,217	$ 23,748	$ 22,832	$ 21,092
Accrued interest receivable	9,835		9,996
Financial liabilities:
Deposits	2,924,508		2,685,061
Subordinated debt	33,875		33,799
Mortgagors' escrow accounts	6,251		4,551
Accrued interest payable	806		1,611
Carrying Amount
Financial assets:
Cash and cash equivalents	238,631		105,521
Securities available for sale, at fair value	204,133		209,293
Securities held to maturity, at amortized cost	27,099		44,688
Federal Home Loan Bank stock	13,466		24,969
Loans held for sale	102,121		42,107
Loans, net	3,089,220		2,964,852
Retirement plan annuities	13,231		12,931
Mortgage servicing rights	16,067		22,217
Accrued interest receivable	9,835		9,996
Financial liabilities:
Deposits	2,924,508		2,685,061
Borrowed funds	271,140		519,936
Subordinated debt	33,875		33,799
Mortgagors' escrow accounts	6,251		4,551
Accrued interest payable	806		1,611
Carrying Amount | Derivative loan commitments
Derivative commitments/agreements:
Assets	2,735		1,261
Liabilities	150		112
Carrying Amount | Interest rate swaps
Derivative commitments/agreements:
Assets	20,103		3,193
Liabilities	20,103		3,193
Carrying Amount | Forward loan sale commitments
Derivative commitments/agreements:
Assets	227
Liabilities	296		518
Fair Value
Financial assets:
Cash and cash equivalents	238,631		105,521
Securities available for sale, at fair value	204,133		209,293
Securities held to maturity, at amortized cost	27,099		44,706
Federal Home Loan Bank stock	13,466		24,969
Loans held for sale	102,121		42,107
Loans, net	3,138,453		2,959,333
Retirement plan annuities	13,231		12,931
Mortgage servicing rights	16,067		22,217
Accrued interest receivable	9,835		9,996
Financial liabilities:
Deposits	2,925,230		2,678,989
Borrowed funds	271,937		518,224
Subordinated debt	35,383		34,338
Mortgagors' escrow accounts	6,251		4,551
Accrued interest payable	806		1,611
Fair Value | Derivative loan commitments
Derivative commitments/agreements:
Assets	2,735		1,261
Liabilities	150		112
Fair Value | Interest rate swaps
Derivative commitments/agreements:
Assets	20,103		3,193
Liabilities	20,103		3,193
Fair Value | Forward loan sale commitments
Derivative commitments/agreements:
Assets	227
Liabilities	296		518
Fair Value | Level 1
Financial assets:
Cash and cash equivalents	238,631		105,521
Fair Value | Level 2
Financial assets:
Securities available for sale, at fair value	204,133		209,293
Securities held to maturity, at amortized cost	27,099		44,706
Loans held for sale	102,121		42,107
Mortgage servicing rights	16,067		22,217
Accrued interest receivable	9,835		9,996
Financial liabilities:
Borrowed funds	271,937		518,224
Accrued interest payable	806		1,611
Fair Value | Level 2 | Interest rate swaps
Derivative commitments/agreements:
Assets	20,103		3,193
Liabilities	20,103		3,193
Fair Value | Level 3
Financial assets:
Federal Home Loan Bank stock	13,466		24,969
Loans, net	3,138,453		2,959,333
Retirement plan annuities	13,231		12,931
Financial liabilities:
Deposits	2,925,230		2,678,989
Subordinated debt	35,383		34,338
Mortgagors' escrow accounts	6,251		4,551
Fair Value | Level 3 | Derivative loan commitments
Derivative commitments/agreements:
Assets	2,735		1,261
Liabilities	150		112
Fair Value | Level 3 | Forward loan sale commitments
Derivative commitments/agreements:
Assets	227
Liabilities	$ 296		$ 518