Accumulated Other Comprehensive Income - Components of and Reclassifications from Accumulated Other Comprehensive Income (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ 207,163
Net change in other comprehensive income, net of tax	(544)
Ending balance	199,456
Accumulated other comprehensive income
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	544
Ending balance	0
Unrealized gain on marketable securities, net of tax
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Amount reclassified and recognized in net income or loss, net of tax	17
Unrealized gain reclassified to accumulated deficit, net of tax
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Amount reclassified and recognized in net income or loss, net of tax	(561)
Accumulated currency, net of tax
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Amount reclassified and recognized in net income or loss, net of tax	$ 0