Accumulated Other Comprehensive Income - Components of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income	$ 195,974		$ 207,163	$ 431,523		$ 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	$ 787	$ 6,087	$ 6,661