Note 5 - Earnings Per Share - Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Net income	$ 73,384	$ 46,852
Basic weighted-average number of common shares outstanding (in shares)	79,530,777	79,588,076
Diluted weighted-average number of common shares outstanding (in shares)	79,832,305	79,830,025
Average restricted stock units with anti-dilutive effect (in shares)	71,108	47,399
Basic (in dollars per share)	$ 0.92	$ 0.59
Diluted (in dollars per share)	$ 0.92	$ 0.59
Restricted Stock Units (RSUs) [Member]
Dilutive effect of weighted-average outstanding common share equivalents RSUs (in shares)	301,528	241,949