Note 5 - Earnings Per Share - Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Net income	$ 46,852	$ 66,679
Weighted-average shares:
Basic weighted-average number of common shares outstanding (in shares)	79,588,076	80,455,317
Diluted (in shares)	79,830,025	80,703,134
Average stock options and restricted stock units with anti-dilutive effect (in shares)	47,399	44,545
Basic (in dollars per share)	$ 0.59	$ 0.83
Diluted (in dollars per share)	$ 0.59	$ 0.83
Restricted Stock Units (RSUs) [Member]
Weighted-average shares:
Dilutive effect of weighted-average outstanding common share equivalents RSUs (in shares)	241,949	247,817