Employee Benefit Plans - ESPP (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cash received for the purchase of shares pursuant to the ESPP			$ 100
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued pursuant to the ESPP	233	184	175
Cash received for the purchase of shares pursuant to the ESPP	$ 398	$ 417	$ 322
Weighted average purchase price per share	$ 1.71	$ 2.26	$ 1.84