Securities Available-for-Sale - Additional Information (Detail)	3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($) Security	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) Security	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($) Security
Schedule of Available-for-sale Securities [Line Items]
Securities at fair value pledged to secure public deposits and repurchase agreements	$ 1,185,127,000		$ 1,185,127,000		$ 1,004,743,000
Securities available-for-sale are securities pledged for borrowing	66,454,000		66,454,000		$ 20,056,000
Net gains on sales of securities	$ 45,000	$ 118,000	45,000	$ 118,000
Proceeds from sales of securities available-for-sale			$ 1,946,000	21,215,000
Securities AFS [Member]
Schedule of Available-for-sale Securities [Line Items]
Weighted average remaining life of investment securities available-for-sale			3 years 6 months
Number of securities, temporarily impaired for less than 12 months | Security	37		37		14
Number of securities, temporarily impaired for 12 months or longer | Security	10		10		11
Number of securities, temporarily impaired, total | Security	327		327		290
Federal Home Loan Bank Boston [Member]
Schedule of Available-for-sale Securities [Line Items]
Net gains on sales of securities			$ 33,000	$ 118,000
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	$ 220,489,000		220,489,000		$ 220,482,000
U.S. Government Sponsored Enterprises [Member] | Securities AFS [Member]
Schedule of Available-for-sale Securities [Line Items]
Weighted average remaining life	$ 15,000,000		$ 15,000,000