Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income
Amounts reclassified from other comprehensive income	$ 0	$ 0	$ 0
Period Start Balance	265,431	290,105	211,660
Period End Balance	236,369	265,431	290,105
Unrealized Gain (Loss) on Marketable Securities
Accumulated Other Comprehensive Income
Period Start Balance	(55)
Other comprehensive gain	23
Period End Balance	(32)	(55)
Foreign Currency Items
Accumulated Other Comprehensive Income
Period Start Balance	2,596
Period End Balance	2,596	2,596
Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income
Period Start Balance	2,541	2,307	2,590
Other comprehensive gain	23
Period End Balance	$ 2,564	$ 2,541	$ 2,307