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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Net income	$ 63,899	$ 33,871	$ 102,663	$ 56,120
Less: distributed earnings allocated to participating securities	(96)	(61)	(201)	(128)
Less: undistributed earnings allocated to participating securities	(204)	(98)	(344)	(140)
Net income attributable to common stockholders (numerator for basic earnings per share)	63,599	33,712	102,118	55,852
Add back: undistributed earnings allocated to participating securities	204	98	344	140
Less: undistributed earnings reallocated to participating securities	(201)	(96)	(338)	(138)
Numerator for diluted earnings per share under two class method	$ 63,602	$ 33,714	$ 102,124	$ 55,854
Weighted-average common shares outstanding (in shares)	56,102,684	55,635,454	55,987,525	55,542,325
Denominator for diluted earnings per share under two class method (in shares)	57,041,006	56,639,653	56,968,002	56,231,245
Basic Earnings Per Common Share (in dollars per share)	$ 1.13	$ 0.61	$ 1.82	$ 1.01
Diluted Earnings Per Common Share (in dollars per share)	$ 1.12	$ 0.60	$ 1.79	$ 0.99
Employee Stock Option [Member]
Add: dilutive effect of stock options (in shares)	938,322	1,004,199	980,477	688,920