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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net income	$ 54,725	$ 53,392	$ 63,899	$ 38,764	$ 24,552	$ 61,163	$ 33,871	$ 22,249	$ 210,780	$ 141,835	$ 103,211
Less: distributed earnings allocated to participating securities									(413)	(257)	(163)
Less: undistributed earnings allocated to participating securities									(846)	(441)	(179)
Net income attributable to common stockholders (numerator for basic earnings per share)									209,521	141,137	102,869
Add back: undistributed earnings allocated to participating securities									846	441	179
Less: undistributed earnings reallocated to participating securities									(833)	(431)	(179)
Numerator for diluted earnings per share under two class method									$ 209,534	$ 141,147	$ 102,869
Weighted-average common shares outstanding (in shares)									56,084,373	55,663,908	55,389,898
Denominator for diluted earnings per share under two class method (in shares)									57,250,704	56,901,461	55,562,920
Basic Earnings Per Common Share (in dollars per share)	$ 0.97	$ 0.94	$ 1.13	$ 0.69	$ 0.44	$ 1.09	$ 0.61	$ 0.40	$ 3.74	$ 2.54	$ 1.86
Diluted Earnings Per Common Share (in dollars per share)	$ 0.95	$ 0.93	$ 1.12	$ 0.68	$ 0.43	$ 1.07	$ 0.59	$ 0.39	$ 3.66	$ 2.48	$ 1.85
Employee Stock Option [Member]
Add: dilutive effect of share-based compensation (in shares)									911,181	1,237,553	173,022
Performance Shares [Member]
Add: dilutive effect of share-based compensation (in shares)									255,150