Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Loss before income taxes
U.S. operations	$ 18,544	$ 7,855
Foreign operations	35,577	28,871
Loss before income taxes	54,121	36,726
Net operating loss carry forwards available to offset future federal and state taxable income	143,700	131,600
Gross deferred tax assets:
Net operating loss carryforwards	40,120	36,383
Accrued expenses	200	52
Contributions	4	3
Depreciation	42
Stock-based compensation	2,967	1,640
Research and development and other credits	9,118	6,545
Total gross deferred tax assets	52,451	44,623
Gross deferred tax liabilities:
Depreciation		(18)
Total gross deferred tax liabilities		(18)
Net deferred tax assets	52,451	44,605
Less: valuation allowance	(52,451)	(44,605)
Net deferred tax assets after valuation allowance	0	0
Increase (decrease) in valuation allowance	$ 7,800	$ 2,000
Reconciliation of income tax expense (benefit) at the statutory federal income tax rate and income taxes
Federal income tax expense at statutory rate (as a percent)	21.00%	21.00%	34.00%
Permanent items (as a percent)	(1.00%)	(0.90%)
State income tax, net of federal benefit (as a percent)	2.30%	0.10%
R&D tax credits (as a percent)	4.40%	4.30%
Change in federal statutory rate on deferreds (as a percent)	1.10%	(1.10%)
Foreign income tax effect (as a percent)	(13.80%)	(16.50%)
Other (as a percent)	0.50%	(1.60%)
Change in valuation allowance (as a percent)	(14.50%)	(5.30%)
Effective income tax rate (as a percent)	0.00%	0.00%
Research tax credit carryforward | Federal
Loss before income taxes
Tax credit carryovers	$ 8,400
Research tax credit carryforward | State
Loss before income taxes
Tax credit carryovers	$ 400