Note 8 - Earnings Per Common Share - Basic and Diluted Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net income	$ 15,737	$ 15,257	$ 13,702	$ 10,331	$ 65,074	$ 7,855	$ 9,141	$ 8,214	$ 55,027	$ 90,284	$ 33,464
Denominator:
Basic earnings per common share - weighted-average shares (in shares)									54,590	54,492	54,177
Effect of dilutive stock options (in shares)									559	358	284
Diluted earnings per common share - weighted-average shares and assumed conversions (in shares)									55,149	54,850	54,461
Basic earnings per common share (in dollars per share)	$ 0.29	$ 0.28	$ 0.25	$ 0.19	$ 1.19	$ 0.14	$ 0.17	$ 0.15	$ 1.01	$ 1.66	$ 0.62
Diluted earnings per common share (in dollars per share)	$ 0.29	$ 0.28	$ 0.25	$ 0.19	$ 1.18	$ 0.14	$ 0.17	$ 0.15	$ 1	$ 1.65	$ 0.61