Estimated Fair Values of Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Assets, carrying value:
Cash and cash equivalents	$ 20,510	$ 21,105
Securities available for sale, carrying value	639,845	617,537
Federal Home Loan Bank of Boston and other restricted stock, carrying value	14,045	12,438
Loans - net, carrying value	575,941	546,392
Accrued interest receivable, carrying value	4,683	4,022
Liabilities, carrying value:
Deposits, carrying value	747,551	732,958
Short-term borrowings, carrying value	58,574	52,985
Long-term debt, carrying value	289,970	247,320
Accrued interest payable, carrying value	583	656
Assets, estimated fair value:
Cash and cash equivalents		21,105
Securities available for sale, estimated fair value	639,845	617,537
Federal Home Loan Bank of Boston and other restricted stock, estimated fair value	14,045	12,438
Loans - net, estimated fair value	600,308	552,422
Accrued interest receivable, estimated fair value	4,683	4,022
Liabilities, estimated fair value:
Deposits, estimated fair value	749,782	731,294
Short-term borrowings, estimated fair value	58,574	52,982
Long-term debt, estimated fair value	296,904	258,470
Accrued interest payable, estimated fair value	583	656
Level 1
Assets, estimated fair value:
Cash and cash equivalents	20,510
Securities available for sale, estimated fair value	7,347	6,242
Level 2
Assets, estimated fair value:
Securities available for sale, estimated fair value	632,498	611,295
Liabilities, estimated fair value:
Short-term borrowings, estimated fair value	58,574
Long-term debt, estimated fair value	296,904
Level 3
Assets, estimated fair value:
Federal Home Loan Bank of Boston and other restricted stock, estimated fair value	14,045
Loans - net, estimated fair value	600,308
Accrued interest receivable, estimated fair value	4,683
Liabilities, estimated fair value:
Deposits, estimated fair value	749,782
Accrued interest payable, estimated fair value	$ 583