Note 5 - Earnings Per Share - Basic and Diluted Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net income	$ 92,589	$ 50,580	$ 54,593	$ 40,550	$ 54,725	$ 53,392	$ 63,899	$ 38,764	$ 238,312	$ 210,780	$ 141,835
Less: distributed earnings allocated to participating securities									(466)	(413)	(257)
Less: undistributed earnings allocated to participating securities									(1,020)	(846)	(441)
Net income attributable to common stockholders (numerator for basic earnings per share)									236,826	209,521	141,137
Add back: undistributed earnings allocated to participating securities									1,020	846	441
Less: undistributed earnings reallocated to participating securities									(992)	(833)	(431)
Numerator for diluted earnings per share under two class method									$ 236,854	$ 209,534	$ 141,147
Weighted-average common shares outstanding (in shares)									61,616,988	60,571,123	60,117,021
Denominator for diluted earnings per share under two class method (in shares)									63,702,666	61,830,761	61,453,578
Basic Earnings Per Common Share (in dollars per share)	$ 1.48	$ 0.81	$ 0.88	$ 0.66	$ 0.90	$ 0.87	$ 1.05	$ 0.64	$ 3.84	$ 3.46	$ 2.35
Diluted Earnings Per Common Share (in dollars per share)	$ 1.42	$ 0.79	$ 0.86	$ 0.64	$ 0.88	$ 0.86	$ 1.03	$ 0.63	$ 3.72	$ 3.39	$ 2.30
Share-based Payment Arrangement, Option [Member]
Add: dilutive effect of share-based compensation (in shares)									1,737,945	984,076	1,336,557
Performance Shares [Member]
Add: dilutive effect of share-based compensation (in shares)									347,733	275,562	0