SELECTED QUARTERLY CONSOLIDATED FINANCIAL DATA - Selected quarterly consolidated financial data (Details) - Selected quarterly consolidated financial data - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015
Statement of Income
Interest and dividend income	$ 8,679	$ 8,670	$ 8,522	$ 8,583	$ 8,709	$ 8,664	$ 8,555	$ 8,682
Interest expense	992	9,833	955	919	775	753	754	744
Net interest and dividend income	7,687	7,687	7,567	7,664	7,934	7,911	7,801	7,938
Provision (benefit) for loan losses	503	344	525	463	266	655	196	(200)
Gains on securities, net	427	9	146	2		6	11	175
Trust and Wealth Advisory	821	849	884	784	755	798	890	822
Service charges and fees	856	822	753	702	74	773	759	710
Gains on sales of mortgage loans, net	78	55	57	39	47	47	86	94
Mortgage servicing, net	37	40	45	34	29	18	30	(30)
Other	108	113	116	114	166	115	115	114
Non-interest income	2,327	1,888	2,001	1,675	1,741	1,757	1,891	1,885
Non-interest expense	7,411	6,500	6,640	6,837	6,342	6,205	6,539	6,835
Income before income taxes	2,100	2,731	2,403	2,039	3,067	2,808	2,957	3,188
Income tax provision	580	812	669	527	900	824	886	953
Net income	1,520	1,919	1,734	1,512	2,167	1,984	2,071	2,234
Net income allocated to common stock	1,509	1,904	1,721	1,499	2,127	1,945	2,032	2,194
Financial Condition
Total assets	935,366	928,445	913,494	891,804	891,192	904,233	860,794	865,037
Loans, net	763,184	753,623	749,523	728,845	699,018	687,719	677,726	676,734
Allowance for loan losses	6,127	5,892	5,718	5,877	5,716	5,659	5,059	5,182
Securities	82,834	79,738	83,874	82,151	79,870	83,886	92,932	84,694
Deposits	781,770	786,730	754,471	755,658	754,533	761,479	720,734	724,910
Repurchase agreements	5,535	3,581	3,355	2,620	3,914	4,210	2,771	3,278
FHLBB advances	37,188	27,134	47,083	27,031	26,979	26,928	28,033	28,403
Shareholders' equity	94,007	93,554	92,584	91,402	90,574	105,450	104,104	103,211
Non-performing assets	$ 12,565	$ 14,496	$ 14,579	$ 16,829	$ 16,264	$ 16,602	$ 14,995	$ 14,875
Per Common Share Data
Earnings, basic	$ .55	$ .70	$ .63	$ .55	$ 0.78	$ 0.71	$ 0.74	$ 0.81
Earnings, diluted	.55	.69	.63	.55	0.77	0.71	0.74	0.80
Cash dividends declared	.28	.28	.28	.28	0.28	0.28	0.28	0.28
Cash dividends paid	.28	.28	.28	.28	0.28	0.28	0.28	0.28
Book value	28.90	28.63	28.28	27.84	33.13	32.72	32.26	31.96
Market price: High	38.15	32.52	32.75	34.43	33.70	31.74	32.30	30.39
Market price: Low	$ 30.92	$ 29.60	$ 29.78	$ 29.51	$ 28.80	$ 28.38	$ 28.80	$ 26.08
Statistical Data
Net interest margin (fully tax equivalent)	3.45%	3.57%	3.71%	3.79%	3.88%	3.91%	4.01%	4.11%
Efficiency ratio (fully tax equivalent)	67.08%	64.13%	66.51%	69.28%	63.64%	60.40%	62.91%	65.45%
Return on average assets	0.65%	0.81%	0.77%	0.68%	0.94%	0.87%	0.94%	1.03%
Return on average shareholders' equity	6.43%	8.20%	7.58%	6.68%	9.34%	8.64%	9.26%	10.22%
Weighted average common shares outstanding, basic	2,737	2,737	2,735	2,723	2,710	2,708	2,706	2,699
Weighted average common shares outstanding, diluted	2,755	2,751	2,749	2,741	2,727	2,724	2,724	2,716