Employee Benefit Plan - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Employee benefit plan
Employer match of each participant's eligible contributions (percent)	50.00%
Vesting period	3 years
Total employer matching contributions to the Plan	$ 250	$ 211	$ 199
Maximum
Employee benefit plan
Employer matching contribution as a percentage of each participant's eligible base salary (percent)	3.00%