Note 4 - Stockholders' Equity - Stock Options Activities (Details) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares
Options outstanding (in shares) | shares	2,388,348
Options outstanding (in dollars per share) | $ / shares	$ 15.243
Options outstanding | $	$ 44,188	$ 30,025
Options outstanding (Year)	5 years 61 days	5 years 314 days
Granted (in shares) | shares
Granted (in dollars per share) | $ / shares
Exercised (in shares) | shares	(563,461)
Exercised (in dollars per share) | $ / shares	$ 10.689
Forfeited (in shares) | shares	(88,536)
Forfeited (in dollars per share) | $ / shares	$ 20.319
Options outstanding (in shares) | shares	1,736,351	2,388,348
Options outstanding (in dollars per share) | $ / shares	$ 16.451	$ 15.243
Options exercisable at the end of the year (in shares) | shares	1,467,826
Options exercisable at the end of the year (in dollars per share) | $ / shares	$ 15.116
Options exercisable at the end of the year | $	$ 39,315
Options exercisable at the end of the year (Year)	4 years 294 days