Note 4 - Earnings Per Common Share - Basic and Diluted Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Numerator:
Net income	$ 13,702	$ 9,141	$ 24,033	$ 72,929	$ 17,355
Denominator:
Basic earnings per common share - weighted-average shares (in shares)	54,613	54,493	54,592		54,459
Effect of dilutive stock options (in shares)	513	309	523		313
Diluted earnings per common share - weighted-average shares and assumed conversions (in shares)	55,126	54,802	55,115		54,772
Basic earnings per common share (in dollars per share)	$ 0.25	$ 0.17	$ 0.44		$ 0.32
Diluted earnings per common share (in dollars per share)	$ 0.25	$ 0.17	$ 0.44		$ 0.32