Commitments and Contingencies - Narrative (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) loan	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Loss Contingencies [Line Items]
Rental expense under leases for equipment and premises	$ 2,900,000	$ 2,800,000	$ 2,800,000
Rental income under leases	$ 532,000	424,000	$ 447,000
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	1
Total unfunded commitments	$ 335,100,000	309,000,000
Reserve for unfunded loan commitments and letters of credit	152,000	$ 122,000
Expenditure commitments due in 12 months	$ 47,000