Note 6 - Stockholders' Equity (Details Textual) - USD ($)	1 Months Ended	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2011
Common Stock [Member] | Private Placement 2011 [Member]
Stock Issued During Period, Shares, New Issues		2,395,700	209,607
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Exercised		2,965,477	260,480
Proceeds from Warrant Exercises		$ 0	$ 0
Common Stock [Member]
Stock Issued During Period, Shares, New Issues	5,520,000
Sale of Stock, Price Per Share	$ 24.75		$ 24.75
Proceeds from Issuance or Sale of Equity	$ 128,199,000
Payments of Stock Issuance Costs	$ 8,421,000
Private Placement 2011 [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights				4,000,000
Class of Warrant or Right, Exercise Price of Warrants or Rights				$ 3.60
Proceeds from Issuance or Sale of Equity			$ 128,205,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Net		530,840	713,381
Proceeds from Stock Options Exercised		$ 4,755,000	$ 6,306,000
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized		$ 65,205,000
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition		2 years 73 days