Note 9 - Convertible Debt (Details Textual) - Convertible Debt [Member] $ in Thousands	1 Months Ended		3 Months Ended
	Sep. 30, 2016 USD ($) $ / shares	Dec. 31, 2015 USD ($) $ / shares	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)
Convertible Notes 2015 [Member]
Debt Instrument, Face Amount		$ 230,000
Debt Instrument, Interest Rate, Stated Percentage		1.125%
Debt Instrument, Convertible, Conversion Ratio		24.8988
Debt Instrument, Convertible, Conversion Price | $ / shares		$ 40.16
Interest Expense, Debt, Total			$ 3,681	$ 3,469
Interest Expense, Debt, Excluding Amortization			640	640
Amortization of Debt Discount (Premium)			2,790	2,595
Amortization of Debt Issuance Costs			251	234
Convertible Debt in Default, Percent Holders to Declare Due and Payable		100.00%
Derivative, Cap Price | $ / shares		52.06
Convertible Notes 2016 [Member]
Debt Instrument, Face Amount	$ 287,500
Debt Instrument, Interest Rate, Stated Percentage	0.75%
Debt Instrument, Convertible, Conversion Ratio	17.7508
Debt Instrument, Convertible, Conversion Price | $ / shares	$ 56.34
Interest Expense, Debt, Total			4,291	4,034
Interest Expense, Debt, Excluding Amortization			533	533
Amortization of Debt Discount (Premium)			3,473	3,236
Amortization of Debt Issuance Costs			$ 285	$ 265
Convertible Debt in Default, Percent Holders to Declare Due and Payable	100.00%
Derivative, Cap Price | $ / shares	73.03