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, 2018	Dec. 31, 2017	Dec. 31, 2016
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Average amount outstanding during the period	$ 484,090	$ 351,692	$ 275,533
Amount outstanding at end of period	630,000	367,500	352,500
Highest month end balance during period	$ 630,000	$ 417,500	$ 367,500
Weighted-average interest rate at end of period	2.67%	1.57%	0.79%
Weighted-average interest rate during the period	2.23%	1.20%	0.73%