Loans (Summary of Loans) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial	$ 1,219,327		$ 1,124,628
Residential real estate	715,412		700,414
Consumer	321,958		322,117
Total loans	2,256,697	[1]	2,147,159	[1]
Percent of Total Loans	100.00%		100.00%
Loans pledged as collateral, FHLBB	1,100,000		911,500
Loans Receivable, Deferred Income, Originated	39		31
Loans Receivable, Deferred Income, Purchased	76		67
Commercial Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial	693,221	[2]	624,813	[2]
Percent of Total Loans	31.00%		29.00%
Loans pledged as collateral, FHLBB	250,100		107,100
Commercial Construction and Development [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial	25,132	[3]	10,955	[3]
Percent of Total Loans	1.00%		1.00%
Commercial Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial	500,974	[4]	488,860	[4]
Percent of Total Loans	22.00%		22.00%
Loans pledged as collateral, FHLBB	54,300		27,200
Loans Pledged as Collateral, Federal Reserve Bank	32,700		42,100
Residential Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Residential real estate	692,659	[5]	678,582	[5]
Percent of Total Loans	31.00%		32.00%
Loans pledged as collateral, FHLBB	599,600		611,800
Homeowner Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Residential real estate	22,753		21,832
Percent of Total Loans	1.00%		1.00%
Home Equity Lines [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Consumer	227,549	[6]	223,430	[6]
Percent of Total Loans	10.00%		10.00%
Loans pledged as collateral, FHLBB	191,200		165,400
Home Equity Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Consumer	39,452	[6]	43,121	[6]
Percent of Total Loans	2.00%		2.00%
Other Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Consumer	54,957	[7]	55,566	[7]
Percent of Total Loans	2.00%		3.00%
Commercial Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial	1,219,327		1,124,628
Percent of Total Loans	54.00%		52.00%
Residential Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Residential real estate	715,412		700,414
Percent of Total Loans	32.00%		33.00%
Consumer Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Consumer	$ 321,958		$ 322,117
Percent of Total Loans	14.00%		15.00%

[1] Includes net unamortized loan origination costs of $39 thousand and $31 thousand, respectively, and net unamortized premiums on purchased loans of $76 thousand and $67 thousand, respectively, at September 30, 2012 and December 31, 2011.

[2] Amortizing mortgages and lines of credit, primarily secured by income producing property. As of September 30, 2012 and December 31, 2011, $250.1 million and $107.1 million, respectively, of these loans were pledged as collateral for FHLBB borrowings (see Note 8).

[3] Loans for construction of residential and commercial properties and for land development.

[4] Loans to businesses and individuals, a substantial portion of which are fully or partially collateralized by real estate. As of September 30, 2012, $54.3 million and $32.7 million, respectively, of these loans were pledged as collateral for FHLBB borrowings and were collateralized for the discount window at the Federal Reserve Bank. Comparable amounts for December 31, 2011 were $27.2 million and $42.1 million, respectively (see Note 8).

[5] As of September 30, 2012 and December 31, 2011, $599.6 million and $611.8 million, respectively, of these loans were pledged as collateral for FHLBB borrowings (see Note 8).

[6] As of September 30, 2012 and December 31, 2011, $191.2 million and $165.4 million, respectively, of these loans were pledged as collateral for FHLBB borrowings (see Note 8).

[7] Fixed-rate consumer installment loans.