Note 13 - Income Taxes - Effective Income Tax Rate Reconciliation (Details)		12 Months Ended
	Apr. 01, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income tax rate	19.00%	19.00%	19.30%	20.00%
Valuation allowance			17.00%
Her Majesty's Revenue and Customs (HMRC) [Member]
Income tax rate		19.00%	19.30%	20.00%
U.K. research and development credit		1.80%	1.50%	1.90%
Effect of U.S. tax reform – Federal tax rate change			(69.20%)
Permanent items		1.00%	5.90%	(1.90%)
Prior period adjustments		(5.20%)
State taxes		9.20%	9.60%	4.90%
Other		2.00%	4.60%	(1.50%)
Effect of foreign tax rate differential		2.90%	28.00%	13.10%
Uncertain tax positions		(1.70%)
Valuation allowance		121.50%	(5.00%)	(21.60%)
Effective income tax rate		150.50%	(5.30%)	14.90%