Commitments (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Unadvanced Commitment on Impaired Loan
Schedule of Commitments [Line Items]
Estimated losses attributable to unadvanced commitments on impaired loans	$ 5,192	$ 430
Commitments to extend credit
Schedule of Commitments [Line Items]
Off-balance sheet financial instruments, contractual amounts	1,733,591	1,469,413
Commitments to extend credit | Fixed Rate Loan Commitment
Schedule of Commitments [Line Items]
Off-balance sheet financial instruments, contractual amounts	170,000	144,800
Letters of credit
Schedule of Commitments [Line Items]
Off-balance sheet financial instruments, contractual amounts	$ 47,206	$ 47,969
Letters of credit | Maximum
Schedule of Commitments [Line Items]
Remaining term	3 years 5 months
Letters of credit | Minimum
Schedule of Commitments [Line Items]
Remaining term	1 month