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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Income
Interest and dividend income									$ 40,372	$ 35,521	$ 34,454
Interest expense									7,221	4,238	3,849
Net interest and dividend income									33,151	31,283	30,605
Provision (benefit) for loan losses									1,728	1,020	1,835
Trust and Wealth Advisory									3,700	3,477	3,338
Service charges and fees									3,718	3,718	3,133
Gains (Losses) on sales of mortgage loans, net									89	125	229
Mortgage servicing, net									308	255	156
(Losses) Gains on CRA mutual fund									(18)
(Losses) Gains on sales of available -for-sale- securities, net									318	178	584
BOLI income and gains									678	343	353
Other									152	140	98
Non-interest income									8,945	8,236	7,891
Non-interest expense									29,835	29,329	27,387
Income before income taxes									10,533	9,170	9,274
Net income allocated to common stock									8,713	6,201	$ 6,633
Financial Condition
Total assets	$ 1,121,554				$ 986,984				1,121,554	986,984
Loans, net	909,279				801,703				$ 909,279	$ 801,703
Per Common Share Data
Earnings, basic									$ 3.15	$ 2.25	$ 2.43
Earnings, diluted									3.13	2.24	2.41
Cash dividends declared									$ 1.12	$ 1.12	$ 1.12
Statistical Data
Weighted average common shares outstanding, basic									2,763	2,755	2,733
Selected quarterly consolidated financial data
Statement of Income
Interest and dividend income	10,807	$ 10,516	$ 9,749	$ 9,300	9,167	$ 8,882	$ 8,687	$ 8,785
Interest expense	2,192	2,016	1,706	1,306	1,143	1,116	1,026	953
Net interest and dividend income	8,615	8,500	8,043	7,993	8,024	7,766	7,661	7,832
Provision (benefit) for loan losses	558	378	467	325	67	237	364	352
(Losses) gains on securities, net					192		(14)
Trust and Wealth Advisory	921	936	949	894	857	874	892	854
Service charges and fees	1,025	932	892	868	919	935	902	962
Gains (Losses) on sales of mortgage loans, net	51	21	(1)	18	21	25	30	49
Mortgage servicing, net	57	84	84	83	75	104	31	45
(Losses) Gains on CRA mutual fund	8	(6)	(7)	(13)
(Losses) Gains on sales of available -for-sale- securities, net	302		17	(2)
BOLI income and gains	432	83	82	82
Other	28	38	42	45	118	142	110	113
Non-interest income	2,824	2,088	2,058	1,975	2,182	2,080	1,951	2,023
Non-interest expense	7,909	7,329	7,417	7,180	8,052	7,220	6,751	7,306
Income before income taxes	2,972	2,881	2,217	2,463	2,087	2,389	2,497	2,197
Income tax provision	408	537	318	446	1,011	695	615	593
Net income	2,564	2,344	1,899	2,017	1,076	1,694	1,882	1,604
Net income allocated to common stock	2,528	2,311	1,877	1,997	1,064	1,677	1,866	1,594
Financial Condition
Total assets	1,121,554	1,098,715	1,096,780	1,014,934	986,984	979,469	974,806	939,549	$ 1,121,554	$ 986,984
Loans, net	909,279	898,625	872,796	830,370	801,703	784,136	771,850	764,665	909,279	801,703
Allowance for loan losses	7,831	7,745	7,381	7,058	6,776	6,494	6,493	6,285	7,831	6,776
Securities	97,150	101,591	90,870	84,878	82,860	88,546	84,468	80,359	97,150	82,860
Deposits	926,739	902,161	897,481	831,837	815,495	831,989	811,341	772,416	926,739	815,495
Repurchase agreements	4,104	6,658	1,691	3,962	1,668	4,529	2,126	2,350	4,104	1,668
FHLBB advances	67,154	67,596	79,538	62,480	54,422	27,364	47,302	52,745	67,154	54,422
Shareholders' equity	103,459	100,767	99,180	98,097	97,514	97,526	96,545	95,221	103,459	97,514
Non-performing assets	$ 8,324	$ 8,513	$ 6,359	$ 5,761	$ 7,354	$ 12,257	$ 11,690	$ 10,890	$ 8,324	$ 7,354
Per Common Share Data
Earnings, basic	$ 0.91	$ 0.84	$ 0.68	$ 0.72	$ 0.39	$ 0.61	$ 0.68	$ 0.58
Earnings, diluted	0.91	0.83	0.68	0.71	0.38	0.60	0.67	0.58
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	36.86	35.93	35.38	35.20	35.01	35.01	34.66	34.38
Market price: High	43.04	48.44	46.70	49.85	48.35	46.90	41.50	40.00
Market price: Low	$ 35.25	$ 40.36	$ 38.15	$ 44.00	$ 44.00	$ 40.40	$ 38.50	$ 37.30
Statistical Data
Net interest margin (fully tax equivalent)	3.34%	3.29%	3.31%	3.46%	3.58%	3.50%	3.58%	3.69%
Efficiency ratio (fully tax equivalent)	69.13%	66.91%	70.87%	69.35%	64.90%	67.18%	66.56%	68.68%
Return on average assets	0.92%	0.85%	0.69%	0.81%	0.43%	0.69%	0.77%	0.70%
Return on average shareholders' equity	9.99%	9.26%	7.68%	8.33%	4.38%	6.89%	7.82%	6.83%
Weighted average common shares outstanding, basic	2,766	2,764	2,761	2,759	2,757	2,757	2,757	2,749
Weighted average common shares outstanding, diluted	2,779	2,779	2,779	2,780	2,778	2,777	2,775	2,768