Commitments and Contingencies - Narrative (Details)	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) loan	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Loss Contingencies [Line Items]
Rental expense under leases for equipment and premises		$ 2,800,000	$ 2,800,000	$ 1,300,000
Rental income under leases		$ 424,000	447,000	$ 260,000
Limit on stop-loss insurance		1 year
Stop-loss insurance limit per covered individual per year		$ 175,000
Loans repurchased in 2 year interval | loan		3
Total unfunded commitments		$ 309,000,000	300,600,000
Reserve for unfunded loan commitments and letters of credit		122,000	$ 114,000
Expenditure commitments due in 12 months		$ 61,000
Scenario, Forecast | Minimum
Loss Contingencies [Line Items]
Additional expenses expected to be incurred	$ 1,700,000
Scenario, Forecast | Maximum
Loss Contingencies [Line Items]
Additional expenses expected to be incurred	$ 2,000,000