Note 7 - Earnings Per Share - Net Income Per Share Computation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Net income	$ 4,153	$ 4,729	$ 16,426	$ 14,793
Common Class A [Member]
Net income	2,062	2,345	8,151	7,339
Allocation of distributed and undistributed income to unvested restricted stock shareholders	(15)	(21)	(63)	(64)
Net income attributable to common shareholders	$ 2,047	$ 2,324	$ 8,088	$ 7,275
Weighted average common shares outstanding - basic (in shares)	20,788	20,716	20,759	20,712
Net income per share – basic (in dollars per share)	$ 0.10	$ 0.11	$ 0.39	$ 0.35
Net income attributable to common shareholders for basic computation	$ 2,047	$ 2,324	$ 8,088	$ 7,275
Weighted average effect of dilutive securities – stock options (in shares)	952	352	778	305
Denominator for diluted earnings per share – adjusted weighted average shares (in shares)	21,740	21,068	21,537	21,017
Net income per share – diluted (in dollars per share)	$ 0.09	$ 0.11	$ 0.38	$ 0.35
Common Class B [Member]
Net income	$ 2,091	$ 2,384	$ 8,275	$ 7,454
Allocation of distributed and undistributed income to unvested restricted stock shareholders	(15)	(21)	(64)	(64)
Net income attributable to common shareholders	$ 2,076	$ 2,363	$ 8,211	$ 7,390
Weighted average common shares outstanding - basic (in shares)	3,514	3,511	3,514	3,503
Net income per share – basic (in dollars per share)	$ 0.59	$ 0.67	$ 2.34	$ 2.11
Net income attributable to common shareholders for basic computation	$ 2,076	$ 2,363	$ 8,211	$ 7,390
Weighted average effect of dilutive securities – stock options (in shares)	106	45	81	54
Denominator for diluted earnings per share – adjusted weighted average shares (in shares)	3,620	3,556	3,595	3,557
Net income per share – diluted (in dollars per share)	$ 0.57	$ 0.66	$ 2.28	$ 2.08