Note 9 - Share-based Compensation - Stock Option Activity Under Equity Incentive Plans and Director Plan (Details) - Common Stock [Member] - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Outstanding, number of options (in shares)	1,373,209
Outstanding, weighted average exercise price (in dollars per share)	$ 15.99
Granted, number of options (in shares)	100,615
Granted, weighted average exercise price (in dollars per share)	$ 41.64
Exercised, number of options (in shares)	(227,902)
Exercised, weighted average exercise price (in dollars per share)	$ 15.88
Exercised, aggregate intrinsic value	$ 8,280	$ 10,621	$ 2,681
Forfeited, number of options (in shares)
Forfeited, weighted average exercise price (in dollars per share)
Outstanding, number of options (in shares)	1,245,922	1,373,209
Outstanding, weighted average exercise price (in dollars per share)	$ 18.08	$ 15.99
Outstanding, weighted average remaining contractual term (Year)	4 years 164 days
Outstanding, aggregate intrinsic value	$ 59,631
Exercisable, number of options (in shares)	807,537
Weighted average exercise price (in dollars per share)	$ 14.33
Exercisable, weighted average remaining contractual term (Year)	3 years 7 days
Exercisable, aggregate intrinsic value	$ 41,677