Securities Available-for-Sale - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 U.S. Government Agency and Sponsored Enterprises Mortgage Backed Securities [Member]	Dec. 31, 2012 U.S. Government Agency and Sponsored Enterprises Mortgage Backed Securities [Member]	Sep. 30, 2013 Federal Home Loan Bank of Boston [Member]	Dec. 31, 2012 Federal Home Loan Bank of Boston [Member]	Sep. 30, 2013 Securities AFS [Member] Security	Dec. 31, 2012 Securities AFS [Member] Security
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale with an amortized cost	$ 1,012,370,000
securities held-to-maturity at their fair value	987,037,000		987,037,000
Securities at fair value pledged to secure public deposits and repurchase agreements					285,264,000	665,028,000
Securities available-for-sale are securities pledged for borrowing							13,450,000	220,313,000
Realized gross gain	1,001,000	529,000	2,665,000	1,119,000
Proceeds from sales of securities available-for-sale			$ 216,078,000	$ 271,500,000
Weighted average remaining life of investment securities available-for-sale			4 years 10 months 24 days
Number of securities, temporarily impaired for less than 12 months									51	20
Number of securities, temporarily impaired for 12 months or longer									4	7
Number of securities, temporarily impaired, total									245	458