Note 1 - Summary of Significant Accounting Policies - Net Income Per Share Computation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Numerator for net income per share - basic:
Net income	$ 37,260	$ 32,406	$ 30,047
Common Stock [Member]
Numerator for net income per share - basic:
Net income	37,260	32,406
Allocation of distributed and undistributed income to unvested restricted stock shareholders	(57)	(109)
Net income attributable to common shareholders	$ 37,203	$ 32,297
Basic earnings per share:
Weighted average common shares outstanding – basic (in shares)	25,170	24,809
Net income per share - basic (in dollars per share)	$ 1.48	$ 1.30
Diluted earnings per share:
Net income attributable to common shareholders	$ 37,203	$ 32,297
Weighted average common shares outstanding – basic (in shares)	25,170	24,809
Weighted average effect of dilutive securities – stock options (in shares)	526	844
Denominator for diluted earnings per share – adjusted weighted average shares (in shares)	25,696	25,653
Net income per share - diluted (in dollars per share)	$ 1.45	$ 1.26
Common Class A [Member]
Numerator for net income per share - basic:
Net income			25,423
Allocation of distributed and undistributed income to unvested restricted stock shareholders			(82)
Net income attributable to common shareholders			$ 25,341
Basic earnings per share:
Weighted average common shares outstanding – basic (in shares)	25,170	24,809	23,562
Net income per share - basic (in dollars per share)	$ 1.48	$ 1.30	$ 1.08
Diluted earnings per share:
Net income attributable to common shareholders			$ 25,341
Weighted average common shares outstanding – basic (in shares)	25,170	24,809	23,562
Weighted average effect of dilutive securities – stock options (in shares)			886
Denominator for diluted earnings per share – adjusted weighted average shares (in shares)	25,696	25,653	24,448
Net income per share - diluted (in dollars per share)	$ 1.45	$ 1.26	$ 1.04
Common Class B [Member]
Numerator for net income per share - basic:
Net income			$ 4,624
Allocation of distributed and undistributed income to unvested restricted stock shareholders			(18)
Net income attributable to common shareholders			$ 4,606
Basic earnings per share:
Weighted average common shares outstanding – basic (in shares)			3,527
Net income per share - basic (in dollars per share)			$ 1.31
Diluted earnings per share:
Net income attributable to common shareholders			$ 4,606
Weighted average common shares outstanding – basic (in shares)			3,527
Weighted average effect of dilutive securities – stock options (in shares)			101
Denominator for diluted earnings per share – adjusted weighted average shares (in shares)			3,628
Net income per share - diluted (in dollars per share)			$ 1.27