Securities Available-for-Sale (Details Textual) (USD $)	9 Months Ended
	Sep. 30, 2012 Security	Sep. 30, 2011	Dec. 31, 2011 Security
Securities Available-for-Sale (Additional Textual) [Abstract]
Realized gross gain	$ 1,119,000	$ 1,245,000
Proceeds from sales of securities available-for-sale	271,500,000	43,124,000
Weighted average remaining life of investment securities available-for-sale	3 years 6 months
Number of securities, temporarily impaired for less than 12 months	9		60
Number of securities, temporarily impaired for 12 months or longer	7		6
Number of securities, temporarily impaired, total	444		393
Federal Home Loan Bank of Boston [Member]
Securities Available-for-Sale (Textual) [Abstract]
Securities available-for-sale are securities pledged for borrowing	236,894,000		246,036,000
U.S. Government Agency and Sponsored Mortgage Backed Securities [Member]
Securities Available-for-Sale (Textual) [Abstract]
Securities at fair value pledged to secure public deposits and repurchase agreements	518,615,000		488,690,000
Investment securities available-for-sale, callable at the discretion of the issuer	$ 70,037,000