Loans - Narrative (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($) loan	Jun. 30, 2016 USD ($) loan	Jun. 30, 2017 USD ($) loan	Jun. 30, 2016 USD ($) loan	Dec. 31, 2017 loan	Dec. 31, 2016 USD ($) loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans over 90 days past due and still accruing interest | loan					0	0
Unadvanced commitments on impaired loans						$ 0
Specific reserves on restructured loans	$ 733,899.000000		$ 733,899.000000
Unadvanced Commitment on Impaired Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Estimated losses attributable to unadvanced commitments on impaired loans	300,000		300,000			300,000
Non-Covered Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Post-Modification Outstanding Recorded Balance	$ 676,000	$ 2,320,000	$ 676,000	$ 2,609,000
Financing Receivable, Modifications, Number of Contracts | loan	1	2	1	4
Financing Receivable, Gross	$ 3,811,345,000		$ 3,811,345,000			3,119,154,000
Financing Receivable, Modifications, Subsequent Default, Number of Contracts | loan	3		3
Non-Covered Loans | Consumer and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Post-Modification Outstanding Recorded Balance	$ 0	$ 0	$ 0	$ 0
Financing Receivable, Modifications, Number of Contracts | loan	0	0	0	0
Financing Receivable, Gross	$ 151,670,000		$ 151,670,000			$ 156,182,000
Financing Receivable, Modifications, Subsequent Default, Number of Contracts | loan	0		0