Note 9 - 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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator for net income per share - basic:
Net income	$ 6,992	$ 4,153	$ 22,246	$ 16,426
Common Class A [Member]
Numerator for net income per share - basic:
Net income	6,992	2,062	17,622	8,151
Allocation of distributed and undistributed income to unvested restricted stock shareholders	(22)	(15)	(61)	(63)
Net income attributable to common shareholders	$ 6,970	$ 2,047	$ 17,561	$ 8,088
Denominator for net income per share - basic:
Weighted average common shares outstanding - basic (in shares)	24,671	20,788	23,184	20,759
Net income per share – basic (in dollars per share)	$ 0.28	$ 0.10	$ 0.76	$ 0.39
Numerator for net income per share - diluted:
Net income attributable to common shareholders for basic computation	$ 6,970	$ 2,047	$ 17,561	$ 8,088
Weighted average common shares outstanding - basic (in shares)	24,671	20,788	23,184	20,759
Weighted average effect of dilutive securities – stock options (in shares)	855	952	899	778
Denominator for diluted earnings per share – adjusted weighted average shares (in shares)	25,526	21,740	24,083	21,537
Net income per share – diluted (in dollars per share)	$ 0.27	$ 0.09	$ 0.73	$ 0.38
Common Class B [Member]
Numerator for net income per share - basic:
Net income		$ 2,091	$ 4,624	$ 8,275
Allocation of distributed and undistributed income to unvested restricted stock shareholders		(15)	(18)	(64)
Net income attributable to common shareholders		$ 2,076	$ 4,606	$ 8,211
Denominator for net income per share - basic:
Weighted average common shares outstanding - basic (in shares)		3,514	3,527	3,514
Net income per share – basic (in dollars per share)		$ 0.59	$ 1.31	$ 2.34
Numerator for net income per share - diluted:
Net income attributable to common shareholders for basic computation		$ 2,076	$ 4,606	$ 8,211
Weighted average common shares outstanding - basic (in shares)		3,514	3,527	3,514
Weighted average effect of dilutive securities – stock options (in shares)		106	101	81
Denominator for diluted earnings per share – adjusted weighted average shares (in shares)		3,620	3,628	3,595
Net income per share – diluted (in dollars per share)		$ 0.57	$ 1.27	$ 2.28