Note 5 - Stock Option Activity (Details) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 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	135,620
Granted, weighted average exercise price (in dollars per share) | $ / shares	$ 15.23
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	1,588,124
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 12.09
Outstanding, weighted average remaining contractual term	6 years 80 days
Outstanding, aggregate intrinsic value | $	$ 5,731
Exercisable, number of options (in shares) | shares	1,089,942
Weighted average exercise price (in dollars per share) | $ / shares	$ 11.14
Exercisable, weighted average remaining contractual term	5 years 43 days
Exercisable, aggregate intrinsic value | $	$ 4,997
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	22,603
Granted, weighted average exercise price (in dollars per share) | $ / shares	$ 34
Exercised, number of options (in shares) | shares	(24,043)
Exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 15.70
Exercised, aggregate intrinsic value | $	$ 435
Forfeited, number of options (in shares) | shares
Forfeited, weighted average exercise price (in dollars per share) | $ / shares
Outstanding, number of options (in shares) | shares	239,233
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 28.10
Outstanding, weighted average remaining contractual term	6 years 208 days
Outstanding, aggregate intrinsic value | $	$ 1,923
Exercisable, number of options (in shares) | shares	157,150
Weighted average exercise price (in dollars per share) | $ / shares	$ 26.02
Exercisable, weighted average remaining contractual term	5 years 175 days
Exercisable, aggregate intrinsic value | $	$ 1,679