Note 9 - Share-based Compensation - Stock Option Activity Under Equity Incentive Plans and Director Plan (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Common Class A [Member]
Outstanding, number of options (in shares)	1,705,483
Outstanding, weighted average exercise price (in dollars per share)	$ 12.31
Granted, number of options (in shares)	299,917	315,620	261,306
Granted, weighted average exercise price (in dollars per share)	$ 22.13
Exercised, number of options (in shares)	(197,784)
Exercised, weighted average exercise price (in dollars per share)	$ 10.55
Exercised, aggregate intrinsic value	$ 2,681	$ 459	$ 350
Forfeited, number of options (in shares)	(60,982)
Forfeited, weighted average exercise price (in dollars per share)	$ 21.35
Outstanding, number of options (in shares)	1,746,634	1,705,483
Outstanding, weighted average exercise price (in dollars per share)	$ 13.88	$ 12.31
Outstanding, weighted average remaining contractual term (Year)	5 years 142 days
Outstanding, aggregate intrinsic value	$ 40,901
Exercisable, number of options (in shares)	1,274,361
Weighted average exercise price (in dollars per share)	$ 12.14
Exercisable, weighted average remaining contractual term (Year)	4 years 116 days
Exercisable, aggregate intrinsic value	$ 32,060
Common Class B [Member]
Outstanding, number of options (in shares)	250,493
Outstanding, weighted average exercise price (in dollars per share)	$ 29.70
Granted, number of options (in shares)	49,986	52,603	43,551
Granted, weighted average exercise price (in dollars per share)	$ 42.90
Exercised, number of options (in shares)	(13,600)
Exercised, weighted average exercise price (in dollars per share)	$ 27.04
Exercised, aggregate intrinsic value	$ 202	$ 632	$ 151
Forfeited, number of options (in shares)	(10,163)
Forfeited, weighted average exercise price (in dollars per share)	$ 41.53
Outstanding, number of options (in shares)	276,716	250,493
Outstanding, weighted average exercise price (in dollars per share)	$ 31.78	$ 29.70
Outstanding, weighted average remaining contractual term (Year)	5 years 189 days
Outstanding, aggregate intrinsic value	$ 6,719
Exercisable, number of options (in shares)	198,950
Weighted average exercise price (in dollars per share)	$ 29.16
Exercisable, weighted average remaining contractual term (Year)	4 years 160 days
Exercisable, aggregate intrinsic value	$ 5,353