Securities Available-for-Sale - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Realized gross gain	$ 450,000	$ 3,019,000	$ 1,843,000
Proceeds from sales of securities available-for-sale	40,285,000	224,045,000	294,881,000
Weighted average remaining life of investment securities available-for-sale	4 years 1 month 6 days
Securities at floating rate or adjustable rate	346,994,000
Transfer of securities available-for-sale to held-to-maturity		987,037,000
Available-for-sale securities transferred to held-to-maturity securities unrealized loss		25,333,000
Federal Home Loan Bank of Boston [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale are securities pledged for borrowing	24,810,000	12,214,000
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	$ 301,038,000	$ 368,137,000
Securities AFS [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities, temporarily impaired for less than 12 months	3	47
Number of securities, temporarily impaired for 12 months or longer	14	7
Number of securities, temporarily impaired, total	262	234