Borrowings (Narrative - Federal Home Loan Bank Advances) (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Advances payable to FHLB	$ 361,172,000	$ 540,450,000
FHLB advances, amount of advances, extended maturities	361,172,000	540,450,000
FHLB advances, prepayment penalty charge	3,900,000
Unused line of credit with Federal Home Loan Bank	8,000,000
Securities available for sale and held to maturity included in collateral to Federal Home Loan Bank	225,600,000	320,800,000
Loans pledged as collateral, FHLBB	1,100,000,000	911,500,000
Federal Home Loan Bank of Boston [Member]
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Advances payable to FHLB	361,172,000	540,450,000
FHLB advances, amount of advances modified to lower interest rates and extend maturities	113,000,000
FHLB advances, amount of advances prepaid	86,200,000
Federal Home Loan Bank of Boston [Member] | Original Maturity Due 2015 [Member]
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
FHLB advances, amount of advances, extended maturities	$ 72,500,000