Commitments (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Schedule of Commitments [Line Items]
Unadvanced commitments on impaired loans		$ 0
Commitments to extend credit
Schedule of Commitments [Line Items]
Off-balance sheet financial instruments, contractual amounts	$ 1,315,284	1,298,423
Letters of credit
Schedule of Commitments [Line Items]
Off-balance sheet financial instruments, contractual amounts	$ 49,633	73,790
Letters of credit | Maximum
Schedule of Commitments [Line Items]
Remaining term	3 years 3 months
Letters of credit | Minimum
Schedule of Commitments [Line Items]
Remaining term	1 month
Unadvanced Commitment on Impaired Loan
Schedule of Commitments [Line Items]
Estimated losses attributable to unadvanced commitments on impaired loans	$ 350	350
Fixed Rate Loan Commitment | Commitments to extend credit
Schedule of Commitments [Line Items]
Off-balance sheet financial instruments, contractual amounts	$ 91,800	$ 112,000