Consolidated Statements of Income (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
INTEREST INCOME
Loans, taxable	$ 32,198	$ 33,214	$ 34,983
Loans, non-taxable	17,910	16,082	16,432
Securities available-for-sale, taxable	2,601	13,024	22,286
Securities available-for-sale, non-taxable	282	286	340
Federal Home Loan Bank of Boston dividends	283	59	77
Securities held-to-maturity	31,745	16,615	6,746
Federal funds sold, interest-bearing deposits in other banks and short-term investments	352	485	630
Total interest income	85,371	79,765	81,494
INTEREST EXPENSE
Savings and NOW deposits	2,539	2,585	2,250
Money market accounts	2,715	2,472	2,373
Time deposits	4,421	4,777	6,250
Securities sold under agreements to repurchase	391	361	367
Other borrowed funds and subordinated debentures	9,070	8,610	8,300
Total interest expense	19,136	18,805	19,540
Net interest income	66,235	60,960	61,954
Provision for loan losses (Note 6)	2,050	2,710	4,150
Net interest income after provision for loan losses	64,185	58,250	57,804
OTHER OPERATING INCOME
Service charges on deposit accounts	8,063	8,113	7,880
Lockbox fees	3,099	3,079	2,930
Brokerage commissions	302	257	364
Net gains on sales of securities	450	3,019	1,843
Gains on sales of mortgage loans	757	1,564	297
Other income	2,600	2,583	2,551
Total other operating income	15,271	18,615	15,865
OPERATING EXPENSES
Salaries and employee benefits (Note 17)	35,096	35,244	32,943
Occupancy	5,503	5,000	4,695
Equipment	2,329	2,298	2,255
FDIC assessments	1,970	1,790	1,737
Other (Note 20)	11,832	11,480	11,608
Total operating expenses	56,730	55,812	53,238
Income before income taxes	22,726	21,053	20,431
Provision for income taxes (Note 16)	866	1,007	1,392
Net income	21,860	20,046	19,039
Class A Common Stock [Member]
OPERATING EXPENSES
Net income	17,157	15,698	14,877
SHARE DATA (Note 14)
Weighted average number of shares outstanding, basic	3,591,732	3,575,683	3,557,693
Weighted average number of shares outstanding, diluted	5,562,209	5,557,693	5,549,191
Basic earnings per share	$ 4.78	$ 4.39	$ 4.18
Diluted earnings per share	$ 3.93	$ 3.61	$ 3.43
Class B Common Stock [Member]
OPERATING EXPENSES
Net income	$ 4,703	$ 4,348	$ 4,162
SHARE DATA (Note 14)
Weighted average number of shares outstanding, basic	1,969,030	1,980,855	1,990,474
Weighted average number of shares outstanding, diluted	1,969,030	1,980,855	1,990,474
Basic earnings per share	$ 2.39	$ 2.19	$ 2.09
Diluted earnings per share	$ 2.39	$ 2.19	$ 2.09