Note 11 - Share-based compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Common Stock [Member]
Weighted average grant date fair value of stock options granted (in dollars per share)	$ 10.02	$ 5.83	$ 3.62
Exercised, aggregate intrinsic value	$ 10,621	$ 2,681	$ 459
Intrinsic value of stock options vested (in thousands)	2,719	$ 5,258	$ 1,627
Common Class B [Member]
Weighted average grant date fair value of stock options granted (in dollars per share)		$ 3.66	$ 3.90
Exercised, aggregate intrinsic value	$ 5,937	$ 202	$ 632
Intrinsic value of stock options vested (in thousands)		$ 787	$ 535

Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)(2)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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Fair value of options vested. Excludes equity instruments other than options, for example, but not limited to, share units, stock appreciation rights, restricted stock.

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-Publisher FASB
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-Section 50
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