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, 2017	Dec. 31, 2016	Dec. 31, 2015
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Average amount outstanding during the period	$ 351,692	$ 275,533	$ 155,874
Amount outstanding at end of period	367,500	352,500	107,500
Highest month end balance during period	$ 417,500	$ 367,500	$ 229,500
Weighted-average interest rate at end of period	1.57%	0.79%	0.55%
Weighted-average interest rate during the period	1.20%	0.73%	0.38%