Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
ASSETS
Cash and due from banks (Note 2)	$ 62,400	$ 52,877
Federal funds sold and interest-bearing deposits in other banks	173,751	167,847
Total cash and cash equivalents	236,151	220,724
Short-term investments	3,183	3,233
Securities available-for-sale, amortized cost $500,220 in 2016 and $404,977 in 2015 (Notes 3, 9 and 11)	499,297	404,623
Securities held-to-maturity, fair value $1,635,808 in 2016 and $1,438,960 in 2015 (Notes 4 and 11)	1,653,986	1,438,903
Federal Home Loan Bank of Boston, stock at cost	21,042	28,807
Loans, net (Note 5)	1,923,933	1,731,536
Less: allowance for loan losses (Note 6)	24,406	23,075
Net loans	1,899,527	1,708,461
Bank premises and equipment (Note 7)	23,417	24,106
Accrued interest receivable	9,645	8,002
Other assets (Notes 5, 8 and 16)	116,360	110,582
Total assets	4,462,608	3,947,441
LIABILITIES AND STOCKHOLDERS' EQUITY
Demand deposits	689,286	541,955
Savings and NOW deposits	1,304,394	1,070,585
Money market accounts	1,181,179	989,094
Time deposits (Note 10)	478,359	473,426
Total deposits	3,653,218	3,075,060
Securities sold under agreements to repurchase (Note 11)	182,280	197,850
Other borrowed funds (Note 12)	293,000	368,000
Subordinated debentures (Note 12)	36,083	36,083
Other liabilities	57,986	55,904
Total liabilities	4,222,567	3,732,897
Commitments and contingencies (Notes 7, 18 and 19)
Stockholders' equity (Note 15):
Preferred Stock - $1.00 par value; 100,000 shares authorized; no shares issued and outstanding
Additional paid-in capital	12,292	12,292
Retained earnings	243,565	221,232
Stockholders' equity before adjustment of accumulated other comprehensive income (loss)	261,425	239,092
Unrealized losses on securities available-for-sale, net of taxes	(567)	(246)
Unrealized losses on securities transferred to held-to-maturity, net of taxes	(4,084)	(6,896)
Pension liability, net of taxes	(16,733)	(17,406)
Total accumulated other comprehensive loss, net of taxes (Notes 3, 13 and 15)	(21,384)	(24,548)
Total stockholders' equity	240,041	214,544
Total liabilities and stockholders' equity	4,462,608	3,947,441
Class A Common Stock [Member]
Stockholders' equity (Note 15):
Common stock value	3,601	3,601
Total stockholders' equity	3,601	3,601
Class B Common Stock [Member]
Stockholders' equity (Note 15):
Common stock value	1,967	1,967
Total stockholders' equity	$ 1,967	$ 1,967