Note 9 - Convertible Senior Notes - Summary of Convertible Senior Notes (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Nov. 30, 2005
Net carrying value	$ 62,313	$ 62,142
Five Point Eight Seven Five Percent Convertible Senior Notes Due Two Thousand Thirty Five [Member] | Convertible Debt [Member]
Face amount of convertible senior notes	88,280	88,280	$ 300,000
Discount	(25,967)	(26,138)
Net carrying value	62,313	62,142
Carrying amount of equity component included in paid-in capital	108,714	108,714
Excess of instruments’ if-converted values over face principal amounts