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, 2020	Dec. 31, 2019	Dec. 31, 2018
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Average amount outstanding during the period	$ 690,366	$ 718,722	$ 484,090
Amount outstanding at end of period	354,000	897,000	630,000
Highest month end balance during period	$ 985,500	$ 897,000	$ 630,000
Weighted-average interest rate at end of period	0.39%	2.06%	2.67%
Weighted-average interest rate during the period	1.39%	2.59%	2.23%