Note 4 - Earnings Per Share - Basic and Diluted Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Net income	$ 40,550	$ 38,764
Less: distributed earnings allocated to participating securities	(111)	(105)
Less: undistributed earnings allocated to participating securities	(139)	(124)
Net income attributable to common stockholders (numerator for basic earnings per share)	40,300	38,535
Add back: undistributed earnings allocated to participating securities	139	124
Less: undistributed earnings reallocated to participating securities	(136)	(122)
Numerator for diluted earnings per share under two class method	$ 40,303	$ 38,537
Weighted-average common shares outstanding (in shares)	60,939,364	60,340,774
Denominator for diluted earnings per share under two class method (in shares)	62,708,334	61,447,563
Basic Earnings Per Common Share (in dollars per share)	$ 0.66	$ 0.64
Diluted Earnings Per Common Share (in dollars per share)	$ 0.64	$ 0.63
Share-based Payment Arrangement, Option [Member]
Add: dilutive effect of share-based compensation (in shares)	1,217,846	1,106,789
Performance Shares [Member]
Add: dilutive effect of share-based compensation (in shares)	551,124