Note 6 - 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, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Numerator:
Net income	$ 19,604	$ 18,044	$ 18,134	$ 13,718	$ 15,748	$ 16,587	$ 15,190	$ 13,546	$ 69,500	$ 61,071	$ 55,027
Denominator:
Basic earnings per common share - weighted-average shares (in shares)									82,527	81,945	81,885
Effect of dilutive stock options (in shares)									637	725	839
Diluted earnings per common share - weighted-average shares and assumed conversions (in shares)									83,164	82,670	82,724
Basic earnings per common share (in dollars per share)	$ 0.24	$ 0.22	$ 0.22	$ 0.17	$ 0.19	$ 0.20	$ 0.19	$ 0.17	$ 0.84	$ 0.75	$ 0.67
Diluted earnings per common share (in dollars per share)	$ 0.24	$ 0.22	$ 0.22	$ 0.17	$ 0.19	$ 0.20	$ 0.18	$ 0.16	$ 0.84	$ 0.74	$ 0.67