BASIS OF PRESENTATION - Impact of Adoption (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Sep. 30, 2021	Jun. 30, 2021	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	$ 2,562,885	$ 2,534,154
Total allowance for credit losses on loans	19,082	22,448	$ 22,815	$ 17,907	$ 16,509	$ 15,353
Decrease to retained earnings	195,607	209,426
Tax exempt
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	47,862	42,448
Total allowance for credit losses on loans	58	85	91	68	64	67
Home equity
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	100,096	88,002
Total allowance for credit losses on loans	285	804	822	314	318	320
Total commercial loans | Commercial construction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	117,882	152,700
Total allowance for credit losses on loans	824	2,086	2,372	717	532	317
Total commercial loans | Commercial real estate owner occupied
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total allowance for credit losses on loans				1,894		2,368
Total commercial loans | Commercial real estate non-owner occupied
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total allowance for credit losses on loans				6,457	5,926	4,695
Total commercial real estate
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	1,084,381
Total commercial real estate | Commercial real estate owner occupied
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	219,217	253,792
Total allowance for credit losses on loans	1,783	2,719	2,552	1,894	1,524
Total commercial real estate | Commercial real estate non-owner occupied
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	716,776	733,353
Total allowance for credit losses on loans	7,864	5,582	5,604
Total commercial and industrial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	441,069
Total commercial and industrial | Commercial and industrial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	355,684	336,989
Total allowance for credit losses on loans	3,137	5,306	5,225	3,282	3,056	3,262
Total residential real estate | Residential mortgages
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	995,216	917,301
Total allowance for credit losses on loans	5,010	5,792	6,069	5,078	4,991	4,213
Total consumer | Consumer other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	10,152	9,569
Total allowance for credit losses on loans	121	$ 74	$ 80	$ 97	$ 98	$ 111
Post-CECL Adoption Portfolio Segmentation
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	2,562,885
Total allowance for credit losses on loans	24,310
Allowance for credit losses on OBS credit exposures	1,975
Total allowance for credit losses	26,285
Post-CECL Adoption Portfolio Segmentation | Commercial construction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	117,882
Total allowance for credit losses on loans	2,020
Post-CECL Adoption Portfolio Segmentation | Commercial real estate owner occupied
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	219,217
Total allowance for credit losses on loans	2,491
Post-CECL Adoption Portfolio Segmentation | Commercial real estate non-owner occupied
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	716,776
Total allowance for credit losses on loans	5,856
Post-CECL Adoption Portfolio Segmentation | Tax exempt
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	47,862
Total allowance for credit losses on loans	98
Post-CECL Adoption Portfolio Segmentation | Commercial and industrial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	355,684
Total allowance for credit losses on loans	6,133
Post-CECL Adoption Portfolio Segmentation | Residential mortgages
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	995,216
Total allowance for credit losses on loans	6,742
Post-CECL Adoption Portfolio Segmentation | Home equity
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	100,096
Total allowance for credit losses on loans	888
Post-CECL Adoption Portfolio Segmentation | Consumer other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	10,152
Total allowance for credit losses on loans	82
Impact of CECL Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total allowance for credit losses on loans	5,228
Impact of CECL Adoption | Tax exempt
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total allowance for credit losses on loans	40
Impact of CECL Adoption | Home equity
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total allowance for credit losses on loans	603
Impact of CECL Adoption | Total commercial loans | Commercial construction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total allowance for credit losses on loans	1,196
Impact of CECL Adoption | Total commercial real estate | Commercial real estate owner occupied
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total allowance for credit losses on loans	708
Impact of CECL Adoption | Total commercial real estate | Commercial real estate non-owner occupied
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total allowance for credit losses on loans	(2,008)
Impact of CECL Adoption | Total commercial and industrial | Commercial and industrial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total allowance for credit losses on loans	2,996
Impact of CECL Adoption | Total residential real estate | Residential mortgages
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total allowance for credit losses on loans	1,732
Impact of CECL Adoption | Total consumer | Consumer other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total allowance for credit losses on loans	(39)
Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	2,562,885
Total allowance for credit losses on loans	19,082
Allowance for credit losses on OBS credit exposures	359
Total allowance for credit losses	19,441
Accounting Standards Update 2016-13 | Commercial construction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	117,882
Total allowance for credit losses on loans	824
Accounting Standards Update 2016-13 | Commercial real estate owner occupied
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	219,217
Total allowance for credit losses on loans	1,783
Accounting Standards Update 2016-13 | Commercial real estate non-owner occupied
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	716,776
Total allowance for credit losses on loans	7,864
Accounting Standards Update 2016-13 | Tax exempt
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	47,862
Total allowance for credit losses on loans	58
Accounting Standards Update 2016-13 | Commercial and industrial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	355,684
Total allowance for credit losses on loans	3,137
Accounting Standards Update 2016-13 | Residential mortgages
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	995,216
Total allowance for credit losses on loans	5,010
Accounting Standards Update 2016-13 | Home equity
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	100,096
Total allowance for credit losses on loans	285
Accounting Standards Update 2016-13 | Consumer other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	10,152
Total allowance for credit losses on loans	121
Accounting Standards Update 2016-13 | Impact of CECL Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total allowance for credit losses on loans	5,228
Allowance for credit losses on OBS credit exposures	1,616
Total allowance for credit losses	6,844
Total increase in Allowance for credit losses	6,844
Tax effect	(1,602)
Decrease to retained earnings	5,242
Accounting Standards Update 2016-13 | Impact of CECL Adoption | Commercial construction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total allowance for credit losses on loans	1,196
Accounting Standards Update 2016-13 | Impact of CECL Adoption | Commercial real estate owner occupied
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total allowance for credit losses on loans	708
Accounting Standards Update 2016-13 | Impact of CECL Adoption | Commercial real estate non-owner occupied
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total allowance for credit losses on loans	(2,008)
Accounting Standards Update 2016-13 | Impact of CECL Adoption | Tax exempt
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total allowance for credit losses on loans	40
Accounting Standards Update 2016-13 | Impact of CECL Adoption | Commercial and industrial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total allowance for credit losses on loans	2,996
Accounting Standards Update 2016-13 | Impact of CECL Adoption | Residential mortgages
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total allowance for credit losses on loans	1,732
Accounting Standards Update 2016-13 | Impact of CECL Adoption | Home equity
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total allowance for credit losses on loans	603
Accounting Standards Update 2016-13 | Impact of CECL Adoption | Consumer other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total allowance for credit losses on loans	(39)
Accounting Standards Update 2016-13 | As Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	2,562,885
Total allowance for credit losses on loans	19,082
Allowance for credit losses on OBS credit exposures	359
Total allowance for credit losses	19,441
Accounting Standards Update 2016-13 | As Reported | Commercial construction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	131,123
Total allowance for credit losses on loans	1,044
Accounting Standards Update 2016-13 | As Reported | Commercial real estate
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	953,258
Total allowance for credit losses on loans	10,199
Accounting Standards Update 2016-13 | As Reported | Tax exempt
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	63,431
Total allowance for credit losses on loans	80
Accounting Standards Update 2016-13 | As Reported | Commercial and industrial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	377,638
Total allowance for credit losses on loans	3,302
Accounting Standards Update 2016-13 | As Reported | Residential mortgages
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	923,891
Total allowance for credit losses on loans	4,078
Accounting Standards Update 2016-13 | As Reported | Home equity
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	102,464
Total allowance for credit losses on loans	258
Accounting Standards Update 2016-13 | As Reported | Consumer other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	11,080
Total allowance for credit losses on loans	121
Accounting Standards Update 2016-13 | Reclassification to CECL Portfolio Segmentation | Commercial construction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	(13,241)
Total allowance for credit losses on loans	(220)
Accounting Standards Update 2016-13 | Reclassification to CECL Portfolio Segmentation | Commercial real estate
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	(953,258)
Total allowance for credit losses on loans	(10,199)
Accounting Standards Update 2016-13 | Reclassification to CECL Portfolio Segmentation | Commercial real estate owner occupied
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	219,217
Total allowance for credit losses on loans	1,783
Accounting Standards Update 2016-13 | Reclassification to CECL Portfolio Segmentation | Commercial real estate non-owner occupied
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	716,776
Total allowance for credit losses on loans	7,864
Accounting Standards Update 2016-13 | Reclassification to CECL Portfolio Segmentation | Tax exempt
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	(15,569)
Total allowance for credit losses on loans	(22)
Accounting Standards Update 2016-13 | Reclassification to CECL Portfolio Segmentation | Commercial and industrial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	(21,954)
Total allowance for credit losses on loans	(165)
Accounting Standards Update 2016-13 | Reclassification to CECL Portfolio Segmentation | Residential mortgages
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	71,325
Total allowance for credit losses on loans	932
Accounting Standards Update 2016-13 | Reclassification to CECL Portfolio Segmentation | Home equity
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	(2,368)
Total allowance for credit losses on loans	27
Accounting Standards Update 2016-13 | Reclassification to CECL Portfolio Segmentation | Consumer other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total loans	$ (928)