Note 4 - Share-based Compensation - Stock Option Activity (Details) $ / shares in Units, $ in Thousands	6 Months Ended
	Jun. 30, 2017 USD ($) $ / shares shares
Common Class A [Member]
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 13.97
Outstanding, weighted average remaining contractual term (Year)	6 years 87 days
Outstanding, aggregate intrinsic value | $	$ 23,544
Exercisable, number of options (in shares) | shares	1,313,134
Weighted average exercise price (in dollars per share) | $ / shares	$ 12.03
Exercisable, weighted average remaining contractual term (Year)	5 years 83 days
Exercisable, aggregate intrinsic value | $	$ 19,523
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	299,917
Granted, weighted average exercise price (in dollars per share) | $ / shares	$ 22.13
Exercised, number of options (in shares) | shares	(159,011)
Exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 11.07
Exercised, aggregate intrinsic value | $	$ 1,754
Forfeited, number of options (in shares) | shares	(24,982)
Forfeited, weighted average exercise price (in dollars per share) | $ / shares	$ 17.03
Outstanding, number of options (in shares) | shares	1,821,407
Common Class B [Member]
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 31.96
Outstanding, weighted average remaining contractual term (Year)	6 years 175 days
Outstanding, aggregate intrinsic value | $	$ 4,486
Exercisable, number of options (in shares) | shares	200,550
Weighted average exercise price (in dollars per share) | $ / shares	$ 29.06
Exercisable, weighted average remaining contractual term (Year)	5 years 178 days
Exercisable, aggregate intrinsic value | $	$ 3,747
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	49,986
Granted, weighted average exercise price (in dollars per share) | $ / shares	$ 42.90
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	(4,163)
Forfeited, weighted average exercise price (in dollars per share) | $ / shares	$ 37.30
Outstanding, number of options (in shares) | shares	284,316