Note 6 - Share-based Compensation - Components of Share-based Compensation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Restricted Stock Units (RSUs) [Member]
Pretax compensation expense	$ 47,044	$ 38,632	$ 32,797
Tax benefit	11,300	9,337	7,740
Restricted share units, net of tax	35,744	29,295	25,057
Performance Shares [Member]
Pretax compensation expense	13,654	14,692	14,572
Tax benefit	3,280	3,551	3,439
Restricted share units, net of tax	$ 10,374	$ 11,141	$ 11,133