SELECTED QUARTERLY CONSOLIDATED FINANCIAL DATA - Selected quarterly consolidated financial data (Unaudited) (Details) - USD ($) $ / shares in Units, $ 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
Statement of Income
Interest and dividend income									$ 43,413	$ 40,372	$ 35,521
Interest expense									9,301	7,221	4,238
Net interest and dividend income									34,112	33,151	31,283
Provision (benefit) for loan losses									955	1,728	1,020
Trust and Wealth Advisory									3,995	3,700	3,477
Service charges and fees									4,028	3,718	3,718
Gains (Losses) on sales of mortgage loans, net									116	89	125
Mortgage servicing, net									307	308	255
Gains (Losses) on CRA mutual fund									25	(18)
(Losses) Gains on sales of available -for-sale- securities, net									263	318	178
BOLI income and gains									392	678	343
Other									124	152	140
Non-interest income									9,250	8,945	8,236
Non-interest expense									28,912	29,835	29,329
Income before income taxes									13,495	10,533	9,170
Net income allocated to common stock									10,976	8,713	$ 6,201
Financial Condition
Total assets	$ 1,112,448				$ 1,121,554				1,112,448	1,121,554
Loans, net	927,413				909,279				$ 927,413	$ 909,279
Per Common Share Data
Earnings, basic									$ 3.95	$ 3.15	$ 2.25
Earnings, diluted									3.93	3.13	2.24
Cash dividends declared									$ 1.12	$ 1.12	$ 1.12
Statistical Data
Weighted average common shares outstanding, basic									2,782	2,763	2,755
Selected quarterly consolidated financial data
Statement of Income
Interest and dividend income	10,705	$ 11,023	$ 10,832	$ 10,854	10,807	$ 10,516	$ 9,749	$ 9,300
Interest expense	2,040	2,356	2,488	2,417	2,192	2,016	1,706	1,306
Net interest and dividend income	8,665	8,667	8,344	8,437	8,615	8,500	8,043	7,993
Provision (benefit) for loan losses	417	94	151	294	558	378	467	325
Trust and Wealth Advisory	1,022	1,023	1,044	906	921	936	949	894
Service charges and fees	1,092	1,003	1,012	920	1,025	932	892	868
Gains (Losses) on sales of mortgage loans, net	67	42	1	7	51	21	(1)	18
Mortgage servicing, net	75	76	80	76	57	84	84	83
Gains (Losses) on CRA mutual fund	(4)	6	12	11	8	(6)	(7)	(13)
(Losses) Gains on sales of available -for-sale- securities, net		(9)	281	(9)	302		17	(2)
BOLI income and gains	139	86	87	79	432	83	82	82
Other	28	29	31	37	28	38	42	45
Non-interest income	2,419	2,256	2,548	2,027	2,824	2,088	2,058	1,975
Non-interest expense	7,080	7,184	7,439	7,211	7,909	7,329	7,417	7,180
Income before income taxes	3,587	3,645	3,302	2,959	2,972	2,881	2,217	2,463
Income tax provision	578	657	599	525	408	537	318	446
Net income	3,009	2,988	2,703	2,434	2,564	2,344	1,899	2,017
Net income allocated to common stock	2,960	2,940	2,671	2,408	2,528	2,311	1,877	1,997
Financial Condition
Total assets	1,112,448	1,144,240	1,119,212	1,118,925	1,121,554	1,098,715	1,096,780	1,014,934	$ 1,112,448	$ 1,121,554
Loans, net	927,413	915,083	910,573	911,188	909,279	898,625	872,796	830,370	927,413	909,279
Allowance for loan losses	8,895	8,846	8,887	8,750	7,831	7,745	7,381	7,058	8,895	7,831
Securities	95,925	98,270	103,857	102,479	97,150	101,591	90,870	84,878	95,925	97,150
Deposits	919,506	966,178	950,723	941,969	926,739	902,161	897,481	831,837	919,506	926,739
Repurchase agreements	8,530	8,588	6,308	2,951	4,104	6,658	1,691	3,962	8,530	4,104
FHLBB advances	50,887	37,828	32,769	47,712	67,154	67,596	79,538	62,480	50,887	67,154
Shareholders' equity	113,655	111,580	108,948	106,109	103,459	100,767	99,180	98,097	113,655	103,459
Non-performing assets	$ 3,935	$ 5,687	$ 5,463	$ 7,130	$ 8,324	$ 8,513	$ 6,359	$ 5,761	$ 3,935	$ 8,324
Per Common Share Data
Earnings, basic	$ 1.06	$ 1.06	$ 0.96	$ 0.87	$ 0.91	$ 0.84	$ 0.68	$ 0.72
Earnings, diluted	1.06	1.05	0.95	0.86	0.91	0.83	0.68	0.71
Cash dividends declared	0.28	0.28	0.28	0.28	0.28	0.28	0.28	0.28
Cash dividends paid	0.28	0.28	0.28	0.28	0.28	0.28	0.28	0.28
Book value	40.22	39.52	38.59	37.81	36.86	35.93	35.38	35.20
Market price: High	47.05	40.00	42.60	44.20	43.04	48.44	46.70	49.85
Market price: Low	$ 37.42	$ 35.26	$ 37.20	$ 35.25	$ 35.25	$ 40.36	$ 38.15	$ 44.00
Statistical Data
Net interest margin (fully tax equivalent)	3.34%	3.29%	3.19%	3.28%	3.34%	3.29%	3.31%	3.46%
Efficiency ratio (fully tax equivalent)	61.81%	62.90%	65.81%	66.15%	69.13%	66.91%	70.87%	69.35%
Return on average assets	1.07%	1.05%	0.97%	0.89%	0.92%	0.85%	0.69%	0.81%
Return on average shareholders' equity	10.56%	10.73%	10.07%	9.45%	9.99%	9.26%	7.68%	8.33%
Weighted average common shares outstanding, basic	2,781	2,783	2,780	2,777	2,766	2,764	2,761	2,759
Weighted average common shares outstanding, diluted	2,794	2,795	2,793	2,789	2,779	2,779	2,779	2,780