Accumulated Other Comprehensive Loss - Components of Accumulated Other Comprehensive Loss, Net of Tax (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	$ (333,154)	$ (293,735)
Ending balance	(361,888)	(333,154)
Foreign Currency Translation Adjustments [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	1,079	2,742
Net current period other comprehensive income (loss)	194	(1,663)
Reclassifications	0	0
Ending balance	1,273	1,079
Net Unrealized (Gains) Losses on Marketable Securities [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	126	37
Net current period other comprehensive income (loss)	(136)	89
Reclassifications	0	0
Ending balance	(10)	126
Accumulated Other Comprehensive Loss (Income) [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	1,205	2,779
Net current period other comprehensive income (loss)	58	(1,574)
Reclassifications	0	0
Ending balance	$ 1,263	$ 1,205