Note 3 - Share-based Compensation (Details Textual) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2019 USD ($) shares	Jun. 30, 2019 USD ($) shares
Restricted Stock Units (RSUs) [Member]
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total | $	$ 76.5	$ 76.5
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition		3 years 3 months 18 days
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period | shares	0	170
Performance Shares [Member]
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total | $	$ 19.9	$ 19.9
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition		2 years 2 months 12 days
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period | shares	0	51,441