Investment Securities Held-to-Maturity - Additional Information (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
Schedule of Held-to-maturity Securities [Line Items]
Securities pledged for borrowing at the Federal Home Loan Bank of Boston	$ 1,371,118,000	$ 281,924,000
Weighted average remaining life of investment securities held-to-maturity	5 years 2 months 12 days
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	550,098,000	149,366,000
Securities pledged for borrowing at the Federal Home Loan Bank of Boston	275,390,000	17,758,000
Weighted average remaining life	209,956,000
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	$ 411,072,000	$ 103,617,000
Held-to-Maturity Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Number of securities, temporarily impaired for less than 12 months	105	3
Number of securities, temporarily impaired for 12 months or longer	2	1
Number of securities, temporarily impaired, total	279	96