Note 8 - Earnings Per Share - Net Income Per Share Computation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Numerator for net income per share - basic:
Net income	$ 7,306	$ 6,521
Common Class A [Member]
Numerator for net income per share - basic:
Net income	3,630	3,234
Allocation of distributed and undistributed income to unvested restricted stock shareholders	(15)	(27)
Net income attributable to common shareholders	$ 3,615	$ 3,207
Denominator for net income per share - basic:
Weighted average common shares outstanding - basic (in shares)	20,884	20,737
Net income per share – basic (in dollars per share)	$ 0.17	$ 0.15
Numerator for net income per share - diluted:
Net income attributable to common shareholders for basic computation	$ 3,615	$ 3,207
Weighted average common shares outstanding - basic (in shares)	20,884	20,737
Weighted average effect of dilutive securities – stock options (in shares)	953	508
Denominator for diluted earnings per share – adjusted weighted average shares (in shares)	21,837	21,245
Net income per share – diluted (in dollars per share)	$ 0.17	$ 0.15
Common Class B [Member]
Numerator for net income per share - basic:
Net income	$ 3,676	$ 3,287
Allocation of distributed and undistributed income to unvested restricted stock shareholders	(14)	(27)
Net income attributable to common shareholders	$ 3,662	$ 3,260
Denominator for net income per share - basic:
Weighted average common shares outstanding - basic (in shares)	3,527	3,513
Net income per share – basic (in dollars per share)	$ 1.04	$ 0.93
Numerator for net income per share - diluted:
Net income attributable to common shareholders for basic computation	$ 3,662	$ 3,260
Weighted average common shares outstanding - basic (in shares)	3,527	3,513
Weighted average effect of dilutive securities – stock options (in shares)	103	63
Denominator for diluted earnings per share – adjusted weighted average shares (in shares)	3,630	3,576
Net income per share – diluted (in dollars per share)	$ 1.01	$ 0.91