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, 2019	Dec. 31, 2018	Dec. 31, 2017
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Average amount outstanding during the period	$ 718,722	$ 484,090	$ 351,692
Amount outstanding at end of period	897,000	630,000	367,500
Highest month end balance during period	$ 897,000	$ 630,000	$ 417,500
Weighted-average interest rate at end of period	2.06%	2.67%	1.57%
Weighted-average interest rate during the period	2.59%	2.23%	1.20%