Note 5 - Stock Option Activity (Details) $ / shares in Units, $ in Thousands	6 Months Ended
	Jun. 30, 2016 USD ($) $ / shares shares
Common Class A [Member]
Outstanding, number of options (in shares) | shares	1,485,738
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 11.65
Granted, number of options (in shares) | shares	315,620
Granted, weighted average exercise price (in dollars per share) | $ / shares	$ 14.64
Exercised, number of options (in shares) | shares	(33,234)
Exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 5.12
Exercised, aggregate intrinsic value | $	$ 336
Forfeited, number of options (in shares) | shares
Forfeited, weighted average exercise price (in dollars per share) | $ / shares
Outstanding, number of options (in shares) | shares	1,768,124
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 12.30
Outstanding, weighted average remaining contractual term	6 years 135 days
Outstanding, aggregate intrinsic value | $	$ 3,695
Exercisable, number of options (in shares) | shares	1,233,942
Weighted average exercise price (in dollars per share) | $ / shares	$ 11.50
Exercisable, weighted average remaining contractual term	5 years 120 days
Exercisable, aggregate intrinsic value | $	$ 3,488
Common Class B [Member]
Outstanding, number of options (in shares) | shares	240,673
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 26.31
Granted, number of options (in shares) | shares	52,603
Granted, weighted average exercise price (in dollars per share) | $ / shares	$ 36.16
Exercised, number of options (in shares) | shares	(32,343)
Exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 16.66
Exercised, aggregate intrinsic value | $	$ 588
Forfeited, number of options (in shares) | shares
Forfeited, weighted average exercise price (in dollars per share) | $ / shares
Outstanding, number of options (in shares) | shares	260,933
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 29.49
Outstanding, weighted average remaining contractual term	6 years 292 days
Outstanding, aggregate intrinsic value | $	$ 1,501
Exercisable, number of options (in shares) | shares	172,850
Weighted average exercise price (in dollars per share) | $ / shares	$ 27.51
Exercisable, weighted average remaining contractual term	5 years 288 days
Exercisable, aggregate intrinsic value | $	$ 1,335