Consolidated Statements of Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
INTEREST INCOME
Loans, taxable	$ 34,324	$ 32,136	$ 32,198
Loans, non-taxable	23,440	19,992	17,910
Securities available-for-sale, taxable	3,003	1,900	2,601
Securities available-for-sale, non-taxable	1,051	583	282
Federal Home Loan Bank of Boston dividends	966	658	283
Securities held-to-maturity	32,679	34,388	31,745
Federal funds sold, interest-bearing deposits in other banks and short-term investments	1,236	436	352
Total interest income	96,699	90,093	85,371
INTEREST EXPENSE
Savings and NOW deposits	4,020	2,817	2,539
Money market accounts	3,542	3,038	2,715
Time deposits	5,706	4,887	4,421
Securities sold under agreements to repurchase	472	487	391
Other borrowed funds and subordinated debentures	8,877	8,905	9,070
Total interest expense	22,617	20,134	19,136
Net interest income	74,082	69,959	66,235
Provision for loan losses (Note 6)	1,375	200	2,050
Net interest income after provision for loan losses	72,707	69,759	64,185
OTHER OPERATING INCOME
Service charges on deposit accounts	7,907	7,732	8,063
Lockbox fees	3,164	3,211	3,099
Brokerage commissions	315	380	302
Net gains on sales of securities	64	594	450
Gains on sales of mortgage loans	1,331	1,034	757
Other income	3,441	3,042	2,600
Total other operating income	16,222	15,993	15,271
OPERATING EXPENSES
Salaries and employee benefits (Note 17)	40,048	38,596	35,096
Occupancy	6,147	6,116	5,503
Equipment	2,845	2,626	2,329
FDIC assessments	1,902	2,152	1,970
Other (Note 20)	13,815	12,708	11,832
Total operating expenses	64,757	62,198	56,730
Income before income taxes	24,172	23,554	22,726
Provision for income taxes (Note 16)	(362)	533	866
Net income	24,534	23,021	21,860
Class A Common Stock [Member]
OPERATING EXPENSES
Net income	$ 19,270	$ 18,081	$ 17,157
SHARE DATA (Note 14)
Weighted average number of shares outstanding, basic	3,600,729	3,600,729	3,591,732
Weighted average number of shares outstanding, diluted	5,567,909	5,567,909	5,562,209
Basic earnings per share	$ 5.35	$ 5.02	$ 4.78
Diluted earnings per share	$ 4.41	$ 4.13	$ 3.93
Class B Common Stock [Member]
OPERATING EXPENSES
Net income	$ 5,264	$ 4,940	$ 4,703
SHARE DATA (Note 14)
Weighted average number of shares outstanding, basic	1,967,180	1,967,180	1,969,030
Weighted average number of shares outstanding, diluted	1,967,180	1,967,180	1,969,030
Basic earnings per share	$ 2.68	$ 2.51	$ 2.39
Diluted earnings per share	$ 2.68	$ 2.51	$ 2.39