Note 4 - Earnings Per Share - Basic and Diluted Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Employee Stock Option [Member]
Add: dilutive effect of stock options (in shares)	155,537	284,318	10,401	226,710
Net income	$ 26,359	$ 14,778	$ 62,835	$ 43,196
Less: distributed earnings allocated to participating securities	(45)	(25)	(124)	(73)
Less: undistributed earnings allocated to participating securities	(49)	(6)	(83)	(15)
Net income attributable to common stockholders (numerator for basic earnings per share)	26,265	14,747	62,628	43,108
Add back: undistributed earnings allocated to participating securities	49	6	83	15
Less: undistributed earnings reallocated to participating securities	(49)	(6)	(83)	(15)
Numerator for diluted earnings per share under two class method	$ 26,265	$ 14,747	$ 62,628	$ 43,108
Weighted-average common shares outstanding (in shares)	48,854,412	48,785,973	48,844,613	48,756,265
Denominator for diluted earnings per share under two class method (in shares)	49,009,949	49,070,291	48,855,014	48,982,975
Basic Earnings Per Common Share (in dollars per share)	$ 0.54	$ 0.30	$ 1.28	$ 0.88
Diluted Earnings Per Common Share (in dollars per share)	$ 0.54	$ 0.30	$ 1.28	$ 0.88