Income Taxes (Details) - USD ($) $ in Thousands		12 Months Ended
	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income taxes
Tax impact of revaluation of deferred tax assets under the TCJA		$ 16,100
Reduction in valuation allowance under the TCJA		16,100
Loss before income taxes
U.S. operations		8,347	$ 28,643	$ 24,850
Foreign operations		10,551
Loss before income taxes		18,898	28,643	$ 24,850
Net operating loss carry forwards available to offset future federal and state taxable income		128,100	121,800
Proceeds from sale of research and development tax credits		400
Gross deferred tax assets:
Net operating loss carryforwards		35,559	48,654
Accrued expenses		52	55
Contributions		4	6
Deferred expenses		35	58
Stock-based compensation		1,459	1,523
Research and development and other credits		5,536	5,258
Total gross deferred tax assets		42,645	55,554
Gross deferred tax liabilities:
Depreciation		(17)	(3)
Total gross deferred tax liabilities		(17)	(3)
Net deferred tax assets		42,628	55,551
Less: valuation allowance		(42,628)	(55,551)
Net deferred tax assets after valuation allowance		0	0
Increase (decrease) in valuation allowance		$ (12,900)	$ 13,500
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%	34.00%	34.00%	34.00%
Permanent items (as a percent)		(0.60%)	(1.20%)	(1.00%)
State income tax, net of federal benefit (as a percent)		2.80%	7.50%	7.10%
R&D tax credits (as a percent)		1.10%	5.20%	4.20%
Change in federal statutory rate on deferreds (as a percent)		(85.60%)
Foreign income tax effect (as a percent)		(19.00%)
Other (as a percent)		(1.10%)	1.60%	5.00%
Change in valuation allowance (as a percent)		68.40%	(47.10%)	(49.30%)
Effective income tax rate (as a percent)		0.00%	0.00%	0.00%
Research tax credit carryforward | Federal
Loss before income taxes
Tax credit carryovers		$ 5,000
Research tax credit carryforward | State
Loss before income taxes
Tax credit carryovers		$ 400