Note 9 - Share-based compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Common Class A [Member]
Weighted average grant date fair value of stock options granted (in dollars per share)	$ 5.83	$ 3.62	$ 3.49
Exercised, aggregate intrinsic value	$ 2,681	$ 459	$ 350
Intrinsic value of stock options vested (in thousands)	$ 5,258	$ 1,627	$ 1,351
Common Class B [Member]
Weighted average grant date fair value of stock options granted (in dollars per share)	$ 3.66	$ 3.90	$ 5.45
Exercised, aggregate intrinsic value	$ 202	$ 632	$ 151
Intrinsic value of stock options vested (in thousands)	$ 787	$ 535	$ 415