Securities Available-for-Sale - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 U.S. Government Agency and Sponsored Enterprises Mortgage Backed Securities [Member]	Dec. 31, 2012 U.S. Government Agency and Sponsored Enterprises Mortgage Backed Securities [Member]	Jun. 30, 2013 Federal Home Loan Bank of Boston [Member]	Dec. 31, 2012 Federal Home Loan Bank of Boston [Member]	Jun. 30, 2013 U.S. Government Sponsored Enterprises [Member]	Jun. 30, 2013 Securities AFS [Member] Security	Dec. 31, 2012 Securities AFS [Member] Security
Schedule of Available-for-sale Securities [Line Items]
Securities at fair value pledged to secure public deposits and repurchase agreements					$ 741,202,000	$ 665,028,000
Securities available-for-sale are securities pledged for borrowing							308,654,000	220,313,000
Realized gross gain	781,000	442,000	1,664,000	590,000
Proceeds from sales of securities available-for-sale			163,593,000	233,002,000
Investment securities available-for-sale, callable at the discretion of the issuer									$ 209,956,000
Weighted average remaining life of investment securities available-for-sale			5 years 1 month 6 days
Number of securities, temporarily impaired for less than 12 months										131	20
Number of securities, temporarily impaired for 12 months or longer										4	7
Number of securities, temporarily impaired, total										456	458