Note 9 - Earnings Per Share - Net Income Per Share Computation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Numerator for net income per share - basic:
Net income	$ 8,196	$ 7,306
Allocation of distributed and undistributed income to unvested restricted stock shareholders	(28)
Net income attributable to common shareholders	$ 8,168
Denominator for net income per share - basic:
Weighted average common shares outstanding - basic (in shares)	24,766
Net income per share – basic (in dollars per share)	$ 0.33
Numerator for net income per share - diluted:
Net income attributable to common shareholders for basic computation	$ 8,168
Weighted average common shares outstanding - basic (in shares)	24,766
Weighted average effect of dilutive securities – stock options (in shares)	743
Denominator for diluted earnings per share – adjusted weighted average shares (in shares)	25,509
Net income per share – diluted (in dollars per share)	$ 0.32
Common Class A [Member]
Numerator for net income per share - basic:
Net income		3,630
Allocation of distributed and undistributed income to unvested restricted stock shareholders		(15)
Net income attributable to common shareholders		$ 3,615
Denominator for net income per share - basic:
Weighted average common shares outstanding - basic (in shares)		20,884
Net income per share – basic (in dollars per share)		$ 0.17
Numerator for net income per share - diluted:
Net income attributable to common shareholders for basic computation		$ 3,615
Weighted average common shares outstanding - basic (in shares)		20,884
Weighted average effect of dilutive securities – stock options (in shares)		953
Denominator for diluted earnings per share – adjusted weighted average shares (in shares)		21,837
Net income per share – diluted (in dollars per share)		$ 0.17
Common Class B [Member]
Numerator for net income per share - basic:
Net income		$ 3,676
Allocation of distributed and undistributed income to unvested restricted stock shareholders		(14)
Net income attributable to common shareholders		$ 3,662
Denominator for net income per share - basic:
Weighted average common shares outstanding - basic (in shares)		3,527
Net income per share – basic (in dollars per share)		$ 1.04
Numerator for net income per share - diluted:
Net income attributable to common shareholders for basic computation		$ 3,662
Weighted average common shares outstanding - basic (in shares)		3,527
Weighted average effect of dilutive securities – stock options (in shares)		103
Denominator for diluted earnings per share – adjusted weighted average shares (in shares)		3,630
Net income per share – diluted (in dollars per share)		$ 1.01