Note 4 - Earnings Per Share - Basic and Diluted Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Net income	$ 54,593	$ 40,550	$ 63,899	$ 38,764	$ 95,143	$ 102,663
Less: distributed earnings allocated to participating securities	(110)		(96)		(221)	(201)
Less: undistributed earnings allocated to participating securities	(213)		(204)		(352)	(344)
Net income attributable to common stockholders (numerator for basic earnings per share)	54,270		63,599		94,570	102,118
Add back: undistributed earnings allocated to participating securities	213		204		352	344
Less: undistributed earnings reallocated to participating securities	(209)		(201)		(345)	(338)
Numerator for diluted earnings per share under two class method	$ 54,274		$ 63,602		$ 94,577	$ 102,124
Weighted-average common shares outstanding (in shares)	61,336,404		60,590,899		61,138,982	60,466,527
Denominator for diluted earnings per share under two class method (in shares)	63,323,267		61,604,286		63,023,149	61,525,442
Basic Earnings Per Common Share (in dollars per share)	$ 0.88		$ 1.05		$ 1.55	$ 1.69
Diluted Earnings Per Common Share (in dollars per share)	$ 0.86		$ 1.03		$ 1.50	$ 1.66
Share-based Payment Arrangement, Option [Member]
Add: dilutive effect of share-based compensation (in shares)	1,435,739		1,013,387		1,333,043	1,058,915
Performance Shares [Member]
Add: dilutive effect of share-based compensation (in shares)	551,124				551,124