Note 8 - Convertible Debt - Schedule of Interest Expense for Convertible Debt (Details) - Convertible Debt [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Convertible Notes 2015 [Member]
Contractual interest expense	$ 2,586	$ 2,587	$ 2,588
Amortization of debt discount	11,645	10,833	10,079
Amortization of debt issuance costs	1,050	976	907
Total interest expense	15,281	14,396	13,574
Convertible Notes 2016 [Member]
Contractual interest expense	2,152	2,156	2,154
Amortization of debt discount	14,472	13,481	12,559
Amortization of debt issuance costs	1,186	1,104	1,029
Total interest expense	$ 17,810	$ 16,741	$ 15,742