Loans - Narrative (Details)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans over 90 days past due and still accruing interest | loan		0
Unadvanced commitments on impaired loans		$ 0
Specific reserves on restructured loans	$ 1,900,000.0
Restructuring Reserve	8,100,000
Unadvanced Commitment on Impaired Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Estimated losses attributable to unadvanced commitments on impaired loans	400,000	350,000
Non-Covered Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restructuring Reserve	724,000	719,000
Financing Receivable, Gross	4,125,153,000	4,024,221,000
Non-Covered Loans | Consumer and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restructuring Reserve	0	0
Financing Receivable, Gross	$ 128,436,000	$ 137,234,000