Summary of Significant Accounting Policies - Components of Other Non-Operating Income (Expense) Recognized in Condensed Consolidated Financial Statements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Nonoperating Income Expense [Abstract]
Interest Income	$ 1,161	$ 59	$ 20
Foreign currency gains (losses), net	(1,460)	85	(1,395)
Currency translation adjustment upon subsidiary liquidation	0	0	(1,397)
Gain on sale of a private company investment	999	6,750	0
Loss on debt modification / extinguishment	(941)	(194)	0
Other income (expense), net	(57)	910	(1,478)
Other non-operating income (expense), net	$ (298)	$ 7,610	$ (4,250)