Note 11 - Stockholders' Equity and Earnings Per Share - Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net income	$ 70,893	$ 56,794	$ 54,321	$ 46,852	$ 67,377	$ 72,835	$ 72,244	$ 66,679	$ 228,860	$ 279,135	$ 271,885
Basic EPS, income									$ 228,860	$ 279,135	$ 271,885
Basic average common shares outstanding (in shares)									79,584,560	79,999,703	81,131,269
Basic (in dollars per share)	$ 0.89	$ 0.71	$ 0.68	$ 0.59	$ 0.85	$ 0.91	$ 0.90	$ 0.83	$ 2.88	$ 3.49	$ 3.35
Effect of dilutive stock options and RSU (in shares)									193,287	248,190	476,077
Diluted EPS, income									$ 228,860	$ 279,135	$ 271,885
Diluted average common shares outstanding (in shares)									79,777,847	80,247,893	81,607,346
Diluted (in dollars per share)	$ 0.89	$ 0.71	$ 0.68	$ 0.59	$ 0.84	$ 0.91	$ 0.90	$ 0.83	$ 2.87	$ 3.48	$ 3.33