Commitments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Unadvanced Commitment on Impaired Loan
Schedule of Commitments [Line Items]
Estimated losses attributable to unadvanced commitments on impaired loans	$ 5,350	$ 5,720
Commitments to extend credit
Schedule of Commitments [Line Items]
Off-balance sheet financial instruments, contractual amounts	2,043,850	1,946,068
Commitments to extend credit | Fixed Rate Loan Commitment
Schedule of Commitments [Line Items]
Off-balance sheet financial instruments, contractual amounts	178,500	160,600
Letters of credit
Schedule of Commitments [Line Items]
Off-balance sheet financial instruments, contractual amounts	$ 58,093	$ 50,971
Letters of credit | Maximum
Schedule of Commitments [Line Items]
Remaining term	4 years 9 months
Letters of credit | Minimum
Schedule of Commitments [Line Items]
Remaining term	1 month