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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Net income	$ 33,871	$ 26,913	$ 56,120	$ 36,476
Less: distributed earnings allocated to participating securities	(61)	(39)	(128)	(79)
Less: undistributed earnings allocated to participating securities	(98)	(47)	(140)	(36)
Net income attributable to common stockholders (numerator for basic earnings per share)	33,712	26,827	55,852	36,361
Add back: undistributed earnings allocated to participating securities	98	47	140	36
Less: undistributed earnings reallocated to participating securities	(96)	(47)	(138)	(36)
Numerator for diluted earnings per share under two class method	$ 33,714	$ 26,827	$ 55,854	$ 36,361
Weighted-average common shares outstanding (in shares)	51,514,309	51,293,917	51,428,079	51,281,643
Denominator for diluted earnings per share under two class method (in shares)	52,444,123	51,304,829	52,065,968	51,291,359
Basic Earnings Per Common Share (in dollars per share)	$ 0.65	$ 0.52	$ 1.09	$ 0.71
Diluted Earnings Per Common Share (in dollars per share)	$ 0.64	$ 0.52	$ 1.07	$ 0.71
Employee Stock Option [Member]
Add: dilutive effect of stock options (in shares)	929,814	10,912	637,889	9,716