Note 5 - Share-based Compensation - Stock Option Activity Under Equity Incentive Plans and Director Plan (Details) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) $ / shares shares
Common Class A [Member]
Outstanding, number of options (in shares) | shares	1,746,634
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 13.88
Granted, number of options (in shares) | shares	74,516
Granted, weighted average exercise price (in dollars per share) | $ / shares	$ 36.80
Exercised, number of options (in shares) | shares	(74,769)
Exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 7.67
Exercised, aggregate intrinsic value | $	$ 2,014
Forfeited, number of options (in shares) | shares
Forfeited, weighted average exercise price (in dollars per share) | $ / shares
Outstanding, number of options (in shares) | shares	1,746,381
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 15.13
Outstanding, weighted average remaining contractual term (Year)	5 years 193 days
Outstanding, aggregate intrinsic value | $	$ 25,227
Exercisable, number of options (in shares) | shares	1,230,618
Weighted average exercise price (in dollars per share) | $ / shares	$ 12.47
Exercisable, weighted average remaining contractual term (Year)	4 years 113 days
Exercisable, aggregate intrinsic value | $	$ 20,646
Common Class B [Member]
Outstanding, number of options (in shares) | shares	276,716
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 31.78
Granted, number of options (in shares) | shares
Granted, weighted average exercise price (in dollars per share) | $ / shares
Exercised, number of options (in shares) | shares	(9,296)
Exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 17.63
Exercised, aggregate intrinsic value | $	$ 356
Forfeited, number of options (in shares) | shares
Forfeited, weighted average exercise price (in dollars per share) | $ / shares
Outstanding, number of options (in shares) | shares	267,420
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 32.27
Outstanding, weighted average remaining contractual term (Year)	5 years 167 days
Outstanding, aggregate intrinsic value | $	$ 4,696
Exercisable, number of options (in shares) | shares	194,825
Weighted average exercise price (in dollars per share) | $ / shares	$ 29.65
Exercisable, weighted average remaining contractual term (Year)	4 years 149 days
Exercisable, aggregate intrinsic value | $	$ 3,931