Note 6 - Share-based Compensation - Stock Option Activity Under Equity Incentive Plans and Director Plan (Details) $ / shares in Units, $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($) $ / shares shares
Exercised, number of options (in shares) | shares	(276,716)
Exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 31.78
Exercised, aggregate intrinsic value | $	$ 5,937
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	116,276
Granted, weighted average exercise price (in dollars per share) | $ / shares	$ 36.12
Exercised, number of options (in shares) | shares	(344,958)
Exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 13.78
Exercised, aggregate intrinsic value | $	$ 7,119
Forfeited, number of options (in shares) | shares	(21,383)
Forfeited, weighted average exercise price (in dollars per share) | $ / shares	$ 26.18
Outstanding, number of options (in shares) | shares	1,496,569
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 15.46
Outstanding, weighted average remaining contractual term (Year)	5 years 87 days
Outstanding, aggregate intrinsic value | $	$ 32,838
Exercisable, number of options (in shares) | shares	1,104,429
Weighted average exercise price (in dollars per share) | $ / shares	$ 13.29
Exercisable, weighted average remaining contractual term (Year)	4 years 80 days
Exercisable, aggregate intrinsic value | $	$ 26,631
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
Forfeited, number of options (in shares) | shares
Forfeited, weighted average exercise price (in dollars per share) | $ / shares
Outstanding, number of options (in shares) | shares
Outstanding, weighted average exercise price (in dollars per share) | $ / shares
Outstanding, weighted average remaining contractual term (Year)
Outstanding, aggregate intrinsic value | $
Exercisable, number of options (in shares) | shares
Weighted average exercise price (in dollars per share) | $ / shares
Exercisable, weighted average remaining contractual term (Year)
Exercisable, aggregate intrinsic value | $