Note 6 - Share-based Compensation - Stock Option Activity Under Equity Incentive Plans and Director Plan (Details) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($) $ / shares shares
Common Class A [Member]
Outstanding, number of options (in shares)	1,746,634
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 13.88
Granted, number of options (in shares)	116,276
Granted, weighted average exercise price (in dollars per share) | $ / shares	$ 36.12
Exercised, number of options (in shares)	(380,958)
Exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 13.82
Exercised, aggregate intrinsic value | $	$ 8,048
Forfeited, number of options (in shares)	(21,383)
Forfeited, weighted average exercise price (in dollars per share) | $ / shares	$ 26.18
Outstanding, weighted average remaining contractual term (Year)	4 years 335 days
Outstanding, aggregate intrinsic value | $	$ 33,756
Exercisable, number of options (in shares)	1,068,429
Weighted average exercise price (in dollars per share) | $ / shares	$ 13.26
Exercisable, weighted average remaining contractual term (Year)	3 years 306 days
Exercisable, aggregate intrinsic value | $	$ 27,077
Outstanding, number of options (in shares)	1,460,569
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 15.49
Common Class B [Member]
Outstanding, number of options (in shares)	276,716
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 31.78
Granted, number of options (in shares)
Granted, weighted average exercise price (in dollars per share) | $ / shares
Exercised, number of options (in shares)	(9,296)
Exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 31.78
Exercised, aggregate intrinsic value | $	$ 5,937
Forfeited, number of options (in shares)
Forfeited, 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)
Weighted average exercise price (in dollars per share) | $ / shares
Exercisable, weighted average remaining contractual term (Year)
Exercisable, aggregate intrinsic value | $
Exercised/Settled in Recapitalization, number of options (in shares)	(276,716)
Outstanding, number of options (in shares)
Outstanding, weighted average exercise price (in dollars per share) | $ / shares