Note 9 - Share-based compensation - Stock Option Activity (Details) - Common Stock [Member] - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Weighted average grant date fair value of stock options granted (in dollars per share)	$ 18.67	$ 11.99	$ 10.02
Exercised, aggregate intrinsic value	$ 25,912	$ 8,280	$ 10,621
Intrinsic value of stock options vested (in thousands)	$ 1,965	$ 1,891	$ 2,719