Note 4 - Share-based Compensation - Stock Option Activity (Details) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($) $ / shares shares
Common Class A [Member]
Outstanding, number of options (in shares) | shares	1,705,483
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 12.31
Granted, number of options (in shares) | shares	119,917
Granted, weighted average exercise price (in dollars per share) | $ / shares	$ 18.80
Exercised, number of options (in shares) | shares	(97,068)
Exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 13.10
Exercised, aggregate intrinsic value | $	$ 590
Forfeited, number of options (in shares) | shares
Forfeited, weighted average exercise price (in dollars per share) | $ / shares
Outstanding, number of options (in shares) | shares	1,728,332
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 12.72
Outstanding, weighted average remaining contractual term (Year)	6 years
Outstanding, aggregate intrinsic value | $	$ 12,064
Exercisable, number of options (in shares) | shares	1,195,077
Weighted average exercise price (in dollars per share) | $ / shares	$ 11.49
Exercisable, weighted average remaining contractual term (Year)	4 years 277 days
Exercisable, aggregate intrinsic value | $	$ 9,807
Common Class B [Member]
Outstanding, number of options (in shares) | shares	250,493
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 29.70
Granted, number of options (in shares) | shares	19,986
Granted, weighted average exercise price (in dollars per share) | $ / shares	$ 40.54
Exercised, number of options (in shares) | shares	(12,000)
Exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 28.41
Exercised, aggregate intrinsic value | $	$ 142
Forfeited, number of options (in shares) | shares
Forfeited, weighted average exercise price (in dollars per share) | $ / shares
Outstanding, number of options (in shares) | shares	258,479
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 30.60
Outstanding, weighted average remaining contractual term (Year)	6 years 138 days
Outstanding, aggregate intrinsic value | $	$ 2,402
Exercisable, number of options (in shares) | shares	170,550
Weighted average exercise price (in dollars per share) | $ / shares	$ 27.52
Exercisable, weighted average remaining contractual term (Year)	5 years 51 days
Exercisable, aggregate intrinsic value | $	$ 2,140