Income Taxes - Net deferred tax assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets / liabilities:
Accruals and allowances	$ 7,331	$ 6,037
Lease liabilities	3,176
Loss (Gains) on foreign exchange	110	(72)
Net operating loss carryforwards	86,912	76,648
Depreciation and amortization	1,739	2,345
R&D tax credits	21,474	17,536
Stock-based compensation	4,038	5,755
Capitalized commissions	(4,118)	(3,915)
Right-of-use assets	(2,764)
Valuation allowance	(117,721)	(104,201)
Net deferred tax assets	$ 177	$ 133	$ 0