Income Taxes - Reconciliation of Income Tax to Expense (Benefit) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Income tax at federal statutory rate	$ (26,022)	$ (42,846)	$ (23,675)
State taxes, net of federal benefit	(8)	(19)	(65)
Change in valuation allowance	16,949	(11,208)	16,024
U.S. statutory tax rate change	0	36,085	0
Permanent interest disallowed	(35)	(150)	(1,832)
Foreign rate change - Impact on Deferred Taxes	1,961	1,619	521
Other permanent differences	(666)	8,236	630
Research and development tax credits	(51)	(274)	(145)
State tax rate benefit	169	56	578
Foreign rate differential	1,731	10,636	6,122
Stock-based compensation	(1,344)	(2,462)	1,132
Net operating losses surrendered under UK's R&D tax relief scheme	(6,322)	0	0
Credits and other	994	327	(238)
Income tax benefit	$ 0	$ 0	$ (948)