Accumulated Other Comprehensive Income - Components of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income	$ 184,502	$ 207,163	$ 323,960		$ 427,569
Net unrealized gain on available-for-sale investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income		544
Foreign currency translation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income		0
Accumulated other comprehensive income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income	$ 0	$ 544	$ 407	$ 788	$ 6,661