Note 12 - Income Taxes (Details Textual) - USD ($) $ in Thousands			12 Months Ended
	Apr. 01, 2020	Apr. 01, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Effective UK Corporate Tax Rate			20.00%	20.30%	21.50%
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent			34.00%	34.00%	34.00%
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent			17.00%
Deferred Tax Assets, Net of Valuation Allowance			$ 6,077
Research Tax Credit Carryforward [Member]
Income Tax Credits and Adjustments			0
State and Local Jurisdiction [Member]
Operating Loss Carryforwards			$ 112,500	$ 85,800
Her Majesty's Revenue and Customs (HMRC) [Member]
Effective UK Corporate Tax Rate			20.00%	20.30%	21.50%
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent			14.40%	(0.50%)	(0.70%)
Operating Loss Carryforwards			$ 14,200	$ 29,000
Deferred Tax Assets, Net of Valuation Allowance			2,600
Internal Revenue Service (IRS) [Member]
Operating Loss Carryforwards			$ 125,300	$ 95,500
Scenario, Forecast [Member]
Effective UK Corporate Tax Rate	17.00%	19.00%
Earliest Tax Year [Member]
Open Tax Year			2013
Latest Tax Year [Member]
Open Tax Year			2016