Note 3 - Share-based Compensation (Details Textual) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2020 USD ($) shares	Sep. 30, 2020 USD ($) shares
Restricted Stock Units (RSUs) [Member]
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total | $	$ 72.8	$ 72.8
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition (Year)		3 years 1 month 6 days
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period (in shares) | shares	214,206	305,558
Performance Shares [Member]
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total | $	$ 17.3	$ 17.3
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition (Year)		2 years 3 months 18 days
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period (in shares) | shares	72,248	117,118