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			9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Net income	$ 15,257	$ 65,074	$ 7,855	$ 39,290	$ 25,210
Basic earnings per common share - weighted-average shares (in shares)	54,661		54,517	54,615	54,479
Effect of dilutive stock options (in shares)	533		373	536	324
Diluted earnings per common share - weighted-average shares and assumed conversions (in shares)	55,194		54,890	55,151	54,803
Basic earnings per common share (in dollars per share)	$ 0.28		$ 0.14	$ 0.72	$ 0.46
Diluted earnings per common share (in dollars per share)	$ 0.28		$ 0.14	$ 0.71	$ 0.46