Securities Available-for-Sale - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($) Security	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) Security	Sep. 30, 2015 USD ($)	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	$ 1,119,685,000		$ 1,119,685,000		$ 1,004,743,000
Securities available-for-sale are securities at fair value pledged for borrowing	63,351,000		63,351,000		$ 20,056,000
Net gains on sales of securities	$ 19,000	$ 52,000	64,000	$ 170,000
Proceeds from sales of securities available-for-sale			$ 2,376,000	42,716,000
Securities AFS [Member]
Schedule of Available-for-sale Securities [Line Items]
Weighted average remaining life of investment securities available-for-sale			3 years 7 months 6 days
Number of securities, temporarily impaired for less than 12 months | Security	41		41		14
Number of securities, temporarily impaired for 12 months or longer | Security	15		15		11
Number of securities, temporarily impaired, total | Security	282		282		290
Federal Home Loan Bank Boston [Member]
Schedule of Available-for-sale Securities [Line Items]
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	$ 223,246,000		223,246,000		$ 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