Commitments (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Dec. 31, 2015
Schedule of Commitments [Line Items]
Unadvanced commitments on impaired loans	$ 100	$ 0
Commitments to extend credit
Schedule of Commitments [Line Items]
Off-balance sheet financial instruments, contractual amounts	1,067,353	1,140,028
Standby letters of credit
Schedule of Commitments [Line Items]
Off-balance sheet financial instruments, contractual amounts	$ 82,606	54,648
Standby letters of credit | Maximum
Schedule of Commitments [Line Items]
Remaining term	2 years 11 months
Standby 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	$ 114,700	$ 93,900