Financial Instruments With Off-Balance Sheet Risk (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Variable rate
Supply Commitment [Line Items]
Fair Value Disclosure, Off-balance Sheet Risks, Amount, Asset	$ 56,410	$ 103,189
Fixed rate
Supply Commitment [Line Items]
Fair Value Disclosure, Off-balance Sheet Risks, Amount, Asset	79,784	65,563
Total commitments to originate new loans
Supply Commitment [Line Items]
Fair Value Disclosure, Off-balance Sheet Risks, Amount, Asset	136,194	168,752
Unadvanced portion of loans and unused lines of credit
Supply Commitment [Line Items]
Fair Value Disclosure, Off-balance Sheet Risks, Amount, Asset	1,327,140	1,280,648
Standby letters of credit
Supply Commitment [Line Items]
Fair Value Disclosure, Off-balance Sheet Risks, Amount, Asset	46,291	38,391
Forward commitments to sell loans
Supply Commitment [Line Items]
Fair Value Disclosure, Off-balance Sheet Risks, Amount, Asset	$ 16,886	$ 9,663