Accumulated Other Comprehensive Income (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Accumulated Other Comprehensive Income
Period Start Balance	$ 236,369
Period End Balance	134,082
Amounts reclassified from other comprehensive income	0
Unrealized Gain (Loss) on Marketable Securities
Accumulated Other Comprehensive Income
Period Start Balance	(32)
Other comprehensive loss	(5)
Period End Balance	(37)
Foreign Currency Items
Accumulated Other Comprehensive Income
Period Start Balance	2,596
Period End Balance	2,596
Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income
Period Start Balance	2,564
Other comprehensive loss	(5)
Period End Balance	$ 2,559