Note 7 - 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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net income	$ 15,748	$ 16,587	$ 15,190	$ 13,546	$ 15,737	$ 15,257	$ 13,702	$ 10,331	$ 61,071	$ 55,027	$ 90,284
Denominator:
Basic earnings per common share - weighted-average shares (in shares)									54,630	54,590	54,492
Effect of dilutive stock options (in shares)									483	559	358
Diluted earnings per common share - weighted-average shares and assumed conversions (in shares)									55,113	55,149	54,850
Basic earnings per common share (in dollars per share)	$ 0.29	$ 0.30	$ 0.28	$ 0.25	$ 0.29	$ 0.28	$ 0.25	$ 0.19	$ 1.12	$ 1.01	$ 1.66
Diluted earnings per common share (in dollars per share)	$ 0.29	$ 0.30	$ 0.28	$ 0.25	$ 0.29	$ 0.28	$ 0.25	$ 0.19	$ 1.11	$ 1.00	$ 1.65