FAIR VALUE OF ASSETS AND LIABILITIES - Recurring basis (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2024 USD ($) item	Dec. 31, 2023 USD ($) item	Mar. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Assets and liabilities measured on recurring basis
Number of transfers | item	0	0
Assets
Securities available for sale, at fair value	$ 291,008,000	$ 290,151,000
Loans held for sale	16,434,000	19,686,000
Mortgage servicing rights	46,597,000	46,111,000	$ 47,080,000	$ 48,138,000
Recurring
Assets
Securities available for sale, at fair value	291,008,000	290,151,000
Loans held for sale	16,434,000	19,686,000
Mortgage servicing rights	46,597,000	46,111,000
Total assets	385,045,000	384,772,000
Recurring | Derivative loan commitments
Assets
Derivative assets	31,006,000	28,824,000
Liabilities
Derivative liabilities	25,415,000	24,551,000
Recurring | Level 1
Assets
Securities available for sale, at fair value	0	0
Recurring | Level 2
Assets
Securities available for sale, at fair value	291,008,000	290,151,000
Loans held for sale	16,434,000	19,686,000
Mortgage servicing rights	46,597,000	46,111,000
Total assets	384,596,000	384,288,000
Recurring | Level 2 | Derivative loan commitments
Assets
Derivative assets	30,557,000	28,340,000
Liabilities
Derivative liabilities	25,365,000	24,100,000
Recurring | Level 3
Assets
Securities available for sale, at fair value	0	0
Total assets	449,000	484,000
Recurring | Level 3 | Derivative loan commitments
Assets
Derivative assets	449,000	484,000
Liabilities
Derivative liabilities	$ 50,000	$ 451,000