Commitments (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
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	995,715,000	947,424,000
Standby letters of credit
Schedule of Commitments [Line Items]
Off-balance sheet financial instruments, contractual amounts	52,981,000	50,108,000
Standby letters of credit | Maximum
Schedule of Commitments [Line Items]
Remaining term	3 years 2 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	200,000
Fixed Rate Loan Commitment | Commitments to extend credit
Schedule of Commitments [Line Items]
Off-balance sheet financial instruments, contractual amounts	73,000,000	$ 65,900,000