Investment Securities Held-to-Maturity - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2015 USD ($) Security	Dec. 31, 2014 USD ($) Security
Schedule of Held-to-maturity Securities [Line Items]
Weighted average remaining life of investment securities held-to-maturity	4 years 2 months 12 days
Held-to-Maturity Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Number of securities, temporarily impaired for less than 12 months	28	34
Number of securities, temporarily impaired for 12 months or longer	21	48
Number of securities, temporarily impaired, total	340	303
Federal Home Loan Bank of Boston [Member]
Schedule of Held-to-maturity Securities [Line Items]
Securities pledged for borrowing at the Federal Home Loan Bank of Boston | $	$ 465,954,000	$ 458,782,000
U.S. Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Securities at fair value pledged to secure public deposits and repurchase agreements | $	1,028,226,000	$ 868,924,000
Weighted average remaining life | $	$ 118,243,000