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, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Numerator for net income per share - basic:
Net income	$ 8,119	$ 7,393	$ 8,196	$ 6,992	$ 7,948	$ 7,306	$ 23,708	$ 22,246
Allocation of distributed and undistributed income to unvested restricted stock shareholders	(27)						(80)
Net income attributable to common shareholders	$ 8,092						$ 23,628
Denominator for net income per share - basic:
Weighted average common shares outstanding - basic (in shares)	24,827						24,794
Net income per share – basic (in dollars per share)	$ 0.33						$ 0.95
Numerator for net income per share - diluted:
Net income attributable to common shareholders for basic computation	$ 8,092						$ 23,628
Weighted average common shares outstanding - basic (in shares)	24,827						24,794
Weighted average effect of dilutive securities – stock options (in shares)	914						830
Denominator for diluted earnings per share – adjusted weighted average shares (in shares)	25,741						25,624
Net income per share – diluted (in dollars per share)	$ 0.31						$ 0.92
Common Class A [Member]
Numerator for net income per share - basic:
Net income				6,992				17,622
Allocation of distributed and undistributed income to unvested restricted stock shareholders				(22)				(61)
Net income attributable to common shareholders				$ 6,970				$ 17,561
Denominator for net income per share - basic:
Weighted average common shares outstanding - basic (in shares)				24,671				23,184
Net income per share – basic (in dollars per share)				$ 0.28				$ 0.76
Numerator for net income per share - diluted:
Net income attributable to common shareholders for basic computation				$ 6,970				$ 17,561
Weighted average common shares outstanding - basic (in shares)				24,671				23,184
Weighted average effect of dilutive securities – stock options (in shares)				855				899
Denominator for diluted earnings per share – adjusted weighted average shares (in shares)				25,526				24,083
Net income per share – diluted (in dollars per share)				$ 0.27				$ 0.73
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
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)								$ 1.31
Numerator for net income per share - diluted:
Net income attributable to common shareholders for basic computation								$ 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