Borrowings (Certain Information of Federal Home Loan Bank of Boston Advances) (Details) - Federal Home Loan Bank of Boston [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Average amount outstanding during the period	$ 275,533	$ 155,874	$ 70,693
Amount outstanding at end of period	352,500	107,500	200,000
Highest month end balance during period	$ 367,500	$ 229,500	$ 200,000
Weighted-average interest rate at end of period	0.79%	0.55%	0.37%
Weighted-average interest rate during the period	0.73%	0.38%	0.35%