Note 4 - Stockholders' Equity - Stock Options Activities (Details) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares
Options outstanding, Balance (in shares) | shares	1,456,285
Options outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 16.172
Options outstanding, aggregate intrinsic value | $	$ 55,777	$ 47,152
Options outstanding, weighted average remaining contractual life (Year)	4 years 227 days	4 years 330 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross | shares
Granted, weighted average exercise price (in dollars per share) | $ / shares
Exercised (in shares) | shares	(203,474)
Exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 17.596
Forfeited (in shares) | shares	(1,234)
Forfeited, weighted average exercise price (in dollars per share) | $ / shares	$ 22.07
Options outstanding, Balance (in shares) | shares	1,251,577	1,456,285
Options outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 15.935	$ 16.172
Options exercisable at the end of the year (in shares) | shares	1,122,468
Options exercisable at the end of the year, weighted average exercise price (in dollars per share) | $ / shares	$ 15.151
Options exercisable at the end of the year, aggregate intrinsic value | $	$ 50,903
Options exercisable at the end of the year, weighted average remaining contractual life (Year)	4 years 133 days