LOANS AND ALLOWANCE FOR CREDIT LOSSES - Summary of Total Loans (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021 USD ($) loan	Dec. 31, 2020 USD ($) loan	Jun. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 2,534,154	$ 2,562,885
Allowance for credit losses	22,448	19,082	$ 22,815	$ 17,907	$ 16,509	$ 15,353
Net loans	2,511,706	2,543,803
Accrued interest receivable for loans	8,700	11,400
Tax exempt
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	42,448	47,862
Allowance for credit losses	85	58	91	68	64	67
Net loans	42,448
Home equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	88,002	100,096
Allowance for credit losses	$ 804	$ 285	822	314	318	320
Paycheck Protection Program [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans outstanding | loan	404	746
Outstanding principal balance	$ 24,200	$ 53,800
Total commercial loans | Commercial construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	152,700	117,882
Allowance for credit losses	2,086	824	2,372	717	532	317
Net loans	152,700
Total commercial loans | Commercial real estate owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses				1,894		2,368
Total commercial loans | Commercial real estate non-owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses				6,457	5,926	4,695
Total commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans		1,084,381
Total commercial real estate | Commercial real estate owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	253,792	219,217
Allowance for credit losses	2,719	1,783	2,552	1,894	1,524
Net loans	253,792
Total commercial real estate | Commercial real estate non-owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	733,353	716,776
Allowance for credit losses	5,582	7,864	5,604
Net loans	733,353
Total commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans		441,069
Total commercial and industrial | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	336,989	355,684
Allowance for credit losses	5,306	3,137	5,225	3,282	3,056	3,262
Net loans	336,989
Total residential real estate | Residential mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	917,301	995,216
Allowance for credit losses	5,792	5,010	6,069	5,078	4,991	4,213
Total consumer | Consumer other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	9,569	10,152
Allowance for credit losses	$ 74	$ 121	$ 80	$ 97	$ 98	$ 111