Note 9 - Share-based compensation - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Common Stock [Member]
Weighted average grant date fair value of stock options granted (in dollars per share)	$ 11.99	$ 10.02	$ 5.83
Exercised, aggregate intrinsic value	$ 8,280	$ 10,621	$ 2,681
Intrinsic value of stock options vested (in thousands)	$ 1,891	$ 2,719	$ 5,258
Common Class B [Member]
Weighted average grant date fair value of stock options granted (in dollars per share)			$ 3.66
Exercised, aggregate intrinsic value			$ 202
Intrinsic value of stock options vested (in thousands)			$ 787

Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently vested.

No definition available.

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- Definition

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.

+ References

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=116856206&loc=d3e5070-113901

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- Definition

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

+ References

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)(1)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

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