Note 5 - Earnings Per Share - Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Net income	$ 54,321	$ 72,244	$ 101,173	$ 138,923
Weighted-average shares:
Basic weighted-average number of common shares outstanding (in shares)	79,581,097	80,106,329	79,584,587	80,279,859
Diluted (in shares)	79,682,426	80,302,679	79,756,226	80,501,800
Average restricted stock units with anti-dilutive effect (in shares)	126,084	66,339	86,741	55,502
Basic (in dollars per share)	$ 0.68	$ 0.90	$ 1.27	$ 1.73
Diluted (in dollars per share)	$ 0.68	$ 0.90	$ 1.27	$ 1.73
Restricted Stock Units (RSUs) [Member]
Weighted-average shares:
Dilutive effect of weighted-average outstanding common share equivalents RSUs (in shares)	101,329	196,350	171,639	221,941