Consolidated Statements of Income (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
INTEREST INCOME
Loans, taxable	$ 34,983	$ 36,772	$ 40,163
Loans, non-taxable	16,432	11,324	8,271
Securities available-for-sale, taxable	22,286	22,782	18,958
Securities available-for-sale, non-taxable	340	211	391
Federal Home Loan Bank of Boston dividends	77	46
Securities held-to-maturity	6,746	5,816	7,158
Federal funds sold, interest-bearing deposits in other banks and short-term investments	630	1,114	1,642
Total interest income	81,494	78,065	76,583
INTEREST EXPENSE
Savings and NOW deposits	2,250	2,539	4,072
Money market accounts	2,373	2,706	3,942
Time deposits	6,250	9,356	7,914
Securities sold under agreements to repurchase	367	379	573
Other borrowed funds and subordinated debentures	8,300	7,786	8,316
Total interest expense	19,540	22,766	24,817
Net interest income	61,954	55,299	51,766
Provision for loan losses (Note 6)	4,150	4,550	5,575
Net interest income after provision for loan losses	57,804	50,749	46,191
OTHER OPERATING INCOME
Service charges on deposit accounts	7,880	7,885	7,876
Lockbox fees	2,930	2,770	2,911
Brokerage commissions	364	441	230
Net gains on sales of securities	1,843	1,940	1,851
Other income	2,848	3,204	3,131
Total other operating income	15,865	16,240	15,999
OPERATING EXPENSES
Salaries and employee benefits (Note 16)	32,943	29,630	28,398
Occupancy	4,695	4,411	4,037
Equipment	2,255	2,235	2,132
FDIC assessments	1,737	2,025	2,965
Other (Note 19)	11,608	10,441	9,840
Total operating expenses	53,238	48,742	47,372
Income before income taxes	20,431	18,247	14,818
Provision for income taxes (Note 15)	1,392	1,554	1,244
Net income	19,039	16,693	13,574
Class A Common Stock
OPERATING EXPENSES
Net income	14,877	13,023	10,557
SHARE DATA (Note 13)
Weighted average number of shares outstanding, basic	3,557,693	3,543,233	3,521,179
Weighted average number of shares outstanding, diluted	5,549,191	5,541,794	5,535,742
Basic earnings per share	$ 4.18	$ 3.68	$ 3.00
Diluted earnings per share	$ 3.43	$ 3.01	$ 2.45
Class B Common Stock
OPERATING EXPENSES
Net income	$ 4,162	$ 3,670	$ 3,017
SHARE DATA (Note 13)
Weighted average number of shares outstanding, basic	1,990,474	1,997,411	2,012,327
Weighted average number of shares outstanding, diluted	1,990,474	1,997,411	2,012,327
Basic earnings per share	$ 2.09	$ 1.84	$ 1.50
Diluted earnings per share	$ 2.09	$ 1.84	$ 1.50