Note 9 - Share-based Compensation - Stock Option Activity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Common Class A [Member]
Outstanding, number of options (in shares)	1,485,738
Outstanding, weighted average exercise price (in dollars per share)	$ 11.65
Granted, number of options (in shares)	315,620	261,306	204,166
Granted, weighted average exercise price (in dollars per share)	$ 14.64
Exercised, number of options (in shares)	(52,383)
Exercised, weighted average exercise price (in dollars per share)	$ 6.59
Exercised, aggregate intrinsic value	$ 459,000	$ 350,000	$ 1,500,000
Forfeited, number of options (in shares)	(43,492)
Forfeited, weighted average exercise price (in dollars per share)	$ 13.25
Outstanding, number of options (in shares)	1,705,483	1,485,738
Outstanding, weighted average exercise price (in dollars per share)	$ 12.31	$ 11.65
Outstanding, weighted average remaining contractual term (Year)	5 years 317 days
Outstanding, aggregate intrinsic value	$ 11,404,000
Exercisable, number of options (in shares)	1,250,793
Weighted average exercise price (in dollars per share)	$ 11.64
Exercisable, weighted average remaining contractual term (Year)	4 years 350 days
Exercisable, aggregate intrinsic value	$ 9,203,000
Common Class B [Member]
Outstanding, number of options (in shares)	240,673
Outstanding, weighted average exercise price (in dollars per share)	$ 26.31
Granted, number of options (in shares)	52,603	43,551	32,217
Granted, weighted average exercise price (in dollars per share)	$ 36.16
Exercised, number of options (in shares)	(35,534)
Exercised, weighted average exercise price (in dollars per share)	$ 16.88
Exercised, aggregate intrinsic value	$ 632,000	$ 151,000	$ 502,000
Forfeited, number of options (in shares)	(7,249)
Forfeited, weighted average exercise price (in dollars per share)	$ 26.90
Outstanding, number of options (in shares)	250,493	240,673
Outstanding, weighted average exercise price (in dollars per share)	$ 29.70	$ 26.31
Outstanding, weighted average remaining contractual term (Year)	6 years 116 days
Outstanding, aggregate intrinsic value	$ 3,066,000
Exercisable, number of options (in shares)	175,659
Weighted average exercise price (in dollars per share)	$ 27.82
Exercisable, weighted average remaining contractual term (Year)	5 years 156 days
Exercisable, aggregate intrinsic value	$ 2,500,000