LOANS - Allowance for Loan Losses Activity and Allocation to Loan Segments (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Oct. 05, 2018
Activity in the allowance for loan losses
Balance				$ 20,655				$ 18,489	$ 20,655	$ 18,489	$ 16,968
Provision (credit) for loan losses	$ 1,251	$ 889	$ 1,750	857	$ 1,502	$ 632	$ 886	808	4,747	3,828	2,416
Charge-offs									(2,102)	(1,981)	(1,317)
Recoveries									760	319	422
Balance	24,060				20,655				24,060	20,655	18,489
Allocation of the allowance to loan segments
Total loans												$ 3,165,733	$ 2,980,252
Total allowance for loan losses	24,060			20,655	20,655			18,489	24,060	20,655	18,489	24,060	20,655
Impaired loans
Activity in the allowance for loan losses
Balance				1,258					1,258
Balance	1,161				1,258				1,161	1,258
Allocation of the allowance to loan segments
Total loans												44,880	37,048
Total allowance for loan losses	1,161			1,258	1,258				1,161	1,258		1,161	1,258
Non-impaired loans
Activity in the allowance for loan losses
Balance				19,397					19,397
Balance	22,899				19,397				22,899	19,397
Allocation of the allowance to loan segments
Total loans												3,120,853	2,943,204
Total allowance for loan losses	22,899			19,397	19,397				22,899	19,397		22,899	19,397
Residential
Activity in the allowance for loan losses
Balance				3,239				4,000	3,239	4,000	4,963
Provision (credit) for loan losses									(407)	(761)	(985)
Charge-offs									(136)	(50)	(144)
Recoveries									482	50	166
Balance	3,178				3,239				3,178	3,239	4,000
Allocation of the allowance to loan segments
Total loans												1,100,424	1,115,456
Total allowance for loan losses	3,178			3,239	3,239			4,000	3,178	3,239	4,000	3,178	3,239
Residential | Impaired loans
Activity in the allowance for loan losses
Balance				1,205					1,205
Balance	985				1,205				985	1,205
Allocation of the allowance to loan segments
Total loans												27,275	30,720
Total allowance for loan losses	985			1,205	1,205				985	1,205		985	1,205
Residential | Non-impaired loans
Activity in the allowance for loan losses
Balance				2,034					2,034
Balance	2,193				2,034				2,193	2,034
Allocation of the allowance to loan segments
Total loans												1,073,149	1,084,736
Total allowance for loan losses	2,193			2,034	2,034				2,193	2,034		2,193	2,034
Commercial real estate
Activity in the allowance for loan losses
Balance				10,059				7,835	10,059	7,835	7,150
Provision (credit) for loan losses									2,810	2,318	685
Charge-offs										(94)
Recoveries									6
Balance	12,875				10,059				12,875	10,059	7,835
Allocation of the allowance to loan segments
Total loans												1,172,351	934,420
Total allowance for loan losses	12,875			10,059	10,059			7,835	12,875	10,059	7,835	12,875	10,059
Commercial real estate | Impaired loans
Allocation of the allowance to loan segments
Total loans												530	2,502
Commercial real estate | Non-impaired loans
Activity in the allowance for loan losses
Balance				10,059					10,059
Balance	12,875				10,059				12,875	10,059
Allocation of the allowance to loan segments
Total loans												1,171,821	931,918
Total allowance for loan losses	12,875			10,059	10,059				12,875	10,059		12,875	10,059
Commercial Construction
Activity in the allowance for loan losses
Balance				2,707				1,810	2,707	1,810	924
Provision (credit) for loan losses									(181)	897	886
Balance	2,526				2,707				2,526	2,707	1,810
Allocation of the allowance to loan segments
Total loans												153,907	161,660
Total allowance for loan losses	2,526			2,707	2,707			1,810	2,526	2,707	1,810	2,526	2,707
Commercial Construction | Impaired loans
Allocation of the allowance to loan segments
Total loans												11,244
Commercial Construction | Non-impaired loans
Activity in the allowance for loan losses
Balance				2,707					2,707
Balance	2,526				2,707				2,526	2,707
Allocation of the allowance to loan segments
Total loans												142,663	161,660
Total allowance for loan losses	2,526			2,707	2,707				2,526	2,707		2,526	2,707
Commercial
Activity in the allowance for loan losses
Balance				2,286				2,254	2,286	2,254	1,920
Provision (credit) for loan losses									1,744	1,008	452
Charge-offs									(1,075)	(990)	(134)
Recoveries									22	14	16
Balance	2,977				2,286				2,977	2,286	2,254
Allocation of the allowance to loan segments
Total loans												306,282	277,271
Total allowance for loan losses	2,977			2,286	2,286			2,254	2,977	2,286	2,254	2,977	2,286
Commercial | Impaired loans
Activity in the allowance for loan losses
Balance				53					53
Balance	176				53				176	53
Allocation of the allowance to loan segments
Total loans												5,831	3,826
Total allowance for loan losses	176			53	53				176	53		176	53
Commercial | Non-impaired loans
Activity in the allowance for loan losses
Balance				2,233					2,233
Balance	2,801				2,233				2,801	2,233
Allocation of the allowance to loan segments
Total loans												300,451	273,445
Total allowance for loan losses	2,801			2,233	2,233				2,801	2,233		2,801	2,233
Consumer loans
Activity in the allowance for loan losses
Balance				1,154				1,000	1,154	1,000	780
Provision (credit) for loan losses									497	746	1,019
Charge-offs									(891)	(847)	(1,039)
Recoveries									250	255	240
Balance	1,010				1,154				1,010	1,154	1,000
Allocation of the allowance to loan segments
Total loans												432,769	491,445
Total allowance for loan losses	1,010			1,154	1,154			1,000	1,010	1,154	1,000	1,010	1,154
Consumer loans | Non-impaired loans
Activity in the allowance for loan losses
Balance				1,154					1,154
Balance	1,010				1,154				1,010	1,154
Allocation of the allowance to loan segments
Total loans												432,769	491,445
Total allowance for loan losses	1,010			1,154	1,154				1,010	1,154		1,010	1,154
Unallocated
Activity in the allowance for loan losses
Balance				1,210				1,590	1,210	1,590	1,231
Provision (credit) for loan losses									284	(380)	359
Balance	1,494				1,210				1,494	1,210	1,590
Allocation of the allowance to loan segments
Total allowance for loan losses	1,494			1,210	1,210			$ 1,590	1,494	1,210	$ 1,590	1,494	1,210
Unallocated | Non-impaired loans
Activity in the allowance for loan losses
Balance				1,210					1,210
Balance	1,494				1,210				1,494	1,210
Allocation of the allowance to loan segments
Total allowance for loan losses	$ 1,494			$ 1,210	$ 1,210				$ 1,494	$ 1,210		$ 1,494	$ 1,210
Coastway
Allocation of the allowance to loan segments
Unpaid principal balance of purchased credit impaired loans														$ 5,400