Note 9 - Stock Option Activity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Common Class A [Member]
Number of options (in shares)	1,324,520
Weighted average exercise price (in dollars per share)	$ 11.07
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross	261,306	204,166
Weighted average exercise price (in dollars per share)	$ 14.02
Number of options (in shares)	(43,983)
Weighted average exercise price (in dollars per share)	$ 6.67
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value	$ 350,000	$ 1,500,000
Number of options (in shares)	(56,105)
Weighted average exercise price (in dollars per share)	$ 13.12
Number of options (in shares)	1,485,738	1,324,520
Weighted average exercise price (in dollars per share)	$ 11.65	$ 11.07
Weighted average remaining contractual term	6 years
Aggregate intrinsic value	$ 6,686,000
Number of options (in shares)	1,041,990
Weighted average exercise price (in dollars per share)	$ 11.11
Weighted average remaining contractual term	5 years 80 days
Aggregate intrinsic value	$ 5,263,000
Common Class B [Member]
Number of options (in shares)	213,470
Weighted average exercise price (in dollars per share)	$ 24.32
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross	43,551	32,217
Weighted average exercise price (in dollars per share)	$ 34.47
Number of options (in shares)	(7,330)
Weighted average exercise price (in dollars per share)	$ 15.38
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value	$ 151,000	$ 502,000
Number of options (in shares)	(9,018)
Weighted average exercise price (in dollars per share)	$ 27.61
Number of options (in shares)	240,673	213,470
Weighted average exercise price (in dollars per share)	$ 26.31	$ 24.32
Weighted average remaining contractual term	6 years 21 days
Aggregate intrinsic value	$ 2,532,000
Number of options (in shares)	167,663
Weighted average exercise price (in dollars per share)	$ 25.10
Weighted average remaining contractual term	5 years 105 days
Aggregate intrinsic value	$ 2,041,000