Note 9 - Earnings Per Share - Net Income Per Share Computation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator for net income per share - basic:
Net income	$ 7,393	$ 8,196	$ 7,948	$ 7,306	$ 15,589	$ 15,254
Allocation of distributed and undistributed income to unvested restricted stock shareholders	(25)				(53)
Net income attributable to common shareholders	$ 7,368				$ 15,536
Denominator for net income per share - basic:
Weighted average common shares outstanding - basic (in shares)	24,789				24,777
Net income per share – basic (in dollars per share)	$ 0.30				$ 0.63
Numerator for net income per share - diluted:
Net income attributable to common shareholders for basic computation	$ 7,368				$ 15,536
Weighted average common shares outstanding - basic (in shares)	24,789				24,777
Weighted average effect of dilutive securities – stock options (in shares)	797				772
Denominator for diluted earnings per share – adjusted weighted average shares (in shares)	25,586				25,549
Net income per share – diluted (in dollars per share)	$ 0.29				$ 0.61
Common Class A [Member]
Numerator for net income per share - basic:
Net income			7,000			10,630
Allocation of distributed and undistributed income to unvested restricted stock shareholders			(23)			(38)
Net income attributable to common shareholders			$ 6,977			$ 10,592
Denominator for net income per share - basic:
Weighted average common shares outstanding - basic (in shares)			23,957			22,429
Net income per share – basic (in dollars per share)			$ 0.29			$ 0.47
Numerator for net income per share - diluted:
Net income attributable to common shareholders for basic computation			$ 6,977			$ 10,592
Weighted average common shares outstanding - basic (in shares)			23,957			22,429
Weighted average effect of dilutive securities – stock options (in shares)			889			921
Denominator for diluted earnings per share – adjusted weighted average shares (in shares)			24,846			23,350
Net income per share – diluted (in dollars per share)			$ 0.28			$ 0.45
Common Class B [Member]
Numerator for net income per share - basic:
Net income			$ 948			$ 4,624
Allocation of distributed and undistributed income to unvested restricted stock shareholders			(4)			(18)
Net income attributable to common shareholders			$ 944			$ 4,606
Denominator for net income per share - basic:
Weighted average common shares outstanding - basic (in shares)			3,527			3,527
Net income per share – basic (in dollars per share)			$ 0.27			$ 1.31
Numerator for net income per share - diluted:
Net income attributable to common shareholders for basic computation			$ 944			$ 4,606
Weighted average common shares outstanding - basic (in shares)			3,527			3,527
Weighted average effect of dilutive securities – stock options (in shares)			93			101
Denominator for diluted earnings per share – adjusted weighted average shares (in shares)			3,620			3,628
Net income per share – diluted (in dollars per share)			$ 0.26			$ 1.27