Investment Securities Held-to-Maturity - Additional Information (Detail)	3 Months Ended	9 Months Ended
	Sep. 30, 2016 USD ($) Security	Sep. 30, 2016 USD ($) Security	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($) Security
Schedule of Held-to-maturity 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
Realized gross gains from sales of securities held-to-maturity		12,000
Proceeds from sales of securities held-to-maturity		$ 192,000	$ 0
Held-to-Maturity Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Weighted average remaining life of investment securities held-to-maturity	3 years 9 months 18 days
Number of securities, temporarily impaired for less than 12 months | Security	33	33		101
Number of securities, temporarily impaired for 12 months or longer | Security	15	15		26
Number of securities, temporarily impaired, total | Security	357	357		322
Held-to-Maturity Securities [Member] | U.S. Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Weighted average remaining life	$ 76,745,000	$ 76,745,000
Federal Home Loan Bank Boston [Member]
Schedule of Held-to-maturity Securities [Line Items]
Securities pledged for borrowing at the Federal Home Loan Bank	$ 355,602,000	$ 355,602,000		$ 432,965,000