Securities Available-for-Sale - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($) Security	Sep. 30, 2013 USD ($)	Sep. 30, 2015 USD ($) Security	Dec. 31, 2014 USD ($) Security
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale with an amortized cost		$ 1,012,370,000
Securities held-to-maturity at their fair value		$ 987,037,000
Net gains on sales of securities	$ 52,000		$ 170,000
Proceeds from sales of securities available-for-sale	21,501,000		$ 42,716,000
Weighted average remaining life of investment securities available-for-sale			3 years 6 months
Federal Home Loan Bank of Boston [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale are securities pledged for borrowing	21,396,000		$ 21,396,000	$ 24,810,000
Net gains on sales of securities	52,000		170,000
U.S. Government Agency and Sponsored Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities at fair value pledged to secure public deposits and repurchase agreements	$ 213,438,000		$ 213,438,000	$ 301,038,000
Securities AFS [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities, temporarily impaired for less than 12 months | Security	9		9	3
Number of securities, temporarily impaired for 12 months or longer | Security	14		14	14
Number of securities, temporarily impaired, total | Security	285		285	262