Note 5 - Earnings Per Share - Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Net income	$ 56,794	$ 72,835	$ 157,967	$ 211,758
Weighted-average shares:
Basic weighted-average number of common shares outstanding (in shares)	79,628,372	79,736,814	79,599,288	80,096,855
Diluted (in shares)	79,764,318	79,993,830	79,758,943	80,330,616
Average restricted stock units with anti-dilutive effect (in shares)	117,621	32,321	97,110	47,690
Basic (in dollars per share)	$ 0.71	$ 0.91	$ 1.98	$ 2.64
Diluted (in dollars per share)	$ 0.71	$ 0.91	$ 1.98	$ 2.64
Restricted Stock Units (RSUs) [Member]
Weighted-average shares:
Dilutive effect of weighted-average outstanding common share equivalents RSUs (in shares)	135,946	257,016	159,655	233,761