Note 10 - Share Capital (Details Textual) $ / shares in Units, $ in Thousands		12 Months Ended
	Aug. 14, 2017 USD ($) shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2018 £ / shares shares	Dec. 31, 2017 £ / shares shares	Aug. 14, 2017 £ / shares	Aug. 14, 2017 $ / shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period		694,322	500,182	85,943
Stock Issued During Period, Shares, Restricted Stock Award, Gross		83,378	26,021
Common Stock, Shares, Issued, Total					26,439,334	25,661,634
Common Stock, Shares, Outstanding, Ending Balance					26,439,334	25,661,634
Common Stock, Par or Stated Value Per Share | £ / shares					£ 0.006705	£ 0.006705
Proceeds from Issuance of Common Stock | $			$ 39,298
Common Stock, Shares Authorized					38,978,604	36,183,293
Underwriters [Member]
Stock Issued During Period, Shares, New Issues	2,500,000
Common Stock, Par or Stated Value Per Share | £ / shares							£ 0.006705
Shares Issued, Price Per Share | $ / shares								$ 16.05
Proceeds from Issuance of Common Stock | $	$ 39,300