Note 9 - Share Capital (Details Textual) $ / shares in Units, $ in Thousands		9 Months Ended		12 Months Ended
	Aug. 14, 2017 USD ($) shares	Sep. 30, 2018 USD ($) shares	Sep. 30, 2017 USD ($)	Dec. 31, 2017 £ / shares shares	Sep. 30, 2018 £ / shares	Aug. 14, 2017 £ / shares	Aug. 14, 2017 $ / shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period		613,075		500,182
Stock Issued During Period, Shares, Restricted Stock Award, Gross		63,254		26,021
Common Stock, Par or Stated Value Per Share | £ / shares				£ 0.006705	£ 0.006705
Proceeds from Issuance of Common Stock | $			$ 39,298
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