Accumulated Other Comprehensive Income - Components of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income	$ 199,456	$ 207,163	$ 433,317	$ 427,569
Net unrealized gain on available-for-sale investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income	0	544
Foreign currency translation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income	0	0
Accumulated other comprehensive income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income	$ 0	$ 544	$ 6,087	$ 6,661