Commitments (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
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	$ 1,232,326	1,075,170
Letters of credit
Schedule of Commitments [Line Items]
Off-balance sheet financial instruments, contractual amounts	$ 81,154	78,954
Letters of credit | Maximum
Schedule of Commitments [Line Items]
Remaining term	4 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	$ 300	300
Fixed Rate Loan Commitment | Commitments to extend credit
Schedule of Commitments [Line Items]
Off-balance sheet financial instruments, contractual amounts	$ 119,300	$ 89,700