Note 7 - Leases - Supplemental Cash Flow Information (Details) - USD ($) $ in Thousands		1 Months Ended	6 Months Ended
	Apr. 08, 2019	Mar. 31, 2019	Jun. 30, 2019
UNITED KINGDOM
Non-cash decreases in operating lease liabilities and right-of-use assets as a result of lease modifications		$ (486)	$ (486)
Mansfield, Massachusetts [Member]
Non-cash increases in operating lease liabilities and right-of-use assets as a result of lease modifications	$ 1,819		$ 1,819