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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Net income	$ 61,163	$ 26,359	$ 117,283	$ 62,835
Less: distributed earnings allocated to participating securities	(68)	(45)	(196)	(124)
Less: undistributed earnings allocated to participating securities	(244)	(49)	(375)	(83)
Net income attributable to common stockholders (numerator for basic earnings per share)	60,851	26,265	116,712	62,628
Add back: undistributed earnings allocated to participating securities	244	49	375	83
Less: undistributed earnings reallocated to participating securities	(240)	(49)	(369)	(83)
Numerator for diluted earnings per share under two class method	$ 60,855	$ 26,265	$ 116,718	$ 62,628
Weighted-average common shares outstanding (in shares)	51,650,360	51,297,132	51,502,986	51,286,844
Denominator for diluted earnings per share under two class method (in shares)	52,601,118	51,460,446	52,248,377	51,297,765
Basic Earnings Per Common Share (in dollars per share)	$ 1.18	$ 0.51	$ 2.27	$ 1.22
Diluted Earnings Per Common Share (in dollars per share)	$ 1.16	$ 0.51	$ 2.23	$ 1.22
Employee Stock Option [Member]
Add: dilutive effect of stock options (in shares)	950,758	163,314	745,391	10,921