Accumulated Other Comprehensive Loss - Components of Accumulated Other Comprehensive Loss, Net of Tax (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at December 31, 2017	$ 333,154
Balance at June 30, 2018	365,990
Foreign Currency Translation Adjustments [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at December 31, 2017	1,079
Net current period other comprehensive income	223
Reclassifications	0
Balance at June 30, 2018	1,302
Net Unrealized (Gains) Losses on Marketable Securities [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at December 31, 2017	126
Net current period other comprehensive income	(130)
Reclassifications	0
Balance at June 30, 2018	(4)
Accumulated Other Comprehensive Loss (Income) [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at December 31, 2017	1,205
Net current period other comprehensive income	93
Reclassifications	0
Balance at June 30, 2018	$ 1,298