Note 4 - Stockholders' Equity - Options Outstanding Separated Into Range of Exercise Price (Details)	9 Months Ended
	Sep. 30, 2017 $ / shares shares
Upper range (in shares) | shares	1,736,351
Weighted average remaining contractual life, outstanding (Year)	5 years 61 days
Weighted average exercise price, outstanding (in dollars per share)	$ 16.451
Exercisable (in shares) | shares	1,467,826
Weighted average remaining contractual life, exercisable (Year)	4 years 294 days
Weighted average exercise price, exercisable (in dollars per share)	$ 15.116
Range One [Member]
Lower range (in dollars per share)	1.039
Upper range (in dollars per share)	$ 1.576
Upper range (in shares) | shares	474,009
Weighted average remaining contractual life, outstanding (Year)	1 year 213 days
Weighted average exercise price, outstanding (in dollars per share)	$ 1.30
Exercisable (in shares) | shares	474,009
Weighted average remaining contractual life, exercisable (Year)	1 year 213 days
Weighted average exercise price, exercisable (in dollars per share)	$ 1.30
Range Two [Member]
Lower range (in dollars per share)	6.23
Upper range (in dollars per share)	$ 8.80
Upper range (in shares) | shares	66,929
Weighted average remaining contractual life, outstanding (Year)	4 years 77 days
Weighted average exercise price, outstanding (in dollars per share)	$ 7.072
Exercisable (in shares) | shares	66,929
Weighted average remaining contractual life, exercisable (Year)	4 years 77 days
Weighted average exercise price, exercisable (in dollars per share)	$ 7.072
Range Three [Member]
Lower range (in dollars per share)	12.47
Upper range (in dollars per share)	$ 16.87
Upper range (in shares) | shares	247,589
Weighted average remaining contractual life, outstanding (Year)	6 years 9 days
Weighted average exercise price, outstanding (in dollars per share)	$ 13.359
Exercisable (in shares) | shares	217,379
Weighted average remaining contractual life, exercisable (Year)	5 years 255 days
Weighted average exercise price, exercisable (in dollars per share)	$ 12.871
Range Four [Member]
Lower range (in dollars per share)	19.51
Upper range (in dollars per share)	$ 21.66
Upper range (in shares) | shares	434,529
Weighted average remaining contractual life, outstanding (Year)	6 years 325 days
Weighted average exercise price, outstanding (in dollars per share)	$ 21.215
Exercisable (in shares) | shares	309,082
Weighted average remaining contractual life, exercisable (Year)	6 years 306 days
Weighted average exercise price, exercisable (in dollars per share)	$ 21.211
Range Five [Member]
Lower range (in dollars per share)	22.01
Upper range (in dollars per share)	$ 24.23
Upper range (in shares) | shares	233,602
Weighted average remaining contractual life, outstanding (Year)	6 years 202 days
Weighted average exercise price, outstanding (in dollars per share)	$ 22.261
Exercisable (in shares) | shares	190.129
Weighted average remaining contractual life, exercisable (Year)	6 years 199 days
Weighted average exercise price, exercisable (in dollars per share)	$ 22.296
Range Six [Member]
Upper range (in shares) | shares	140,209
Weighted average remaining contractual life, outstanding (Year)	7 years 144 days
Weighted average exercise price, outstanding (in dollars per share)	$ 29.88
Exercisable (in shares) | shares	87.906
Weighted average remaining contractual life, exercisable (Year)	7 years 144 days
Weighted average exercise price, exercisable (in dollars per share)	$ 29.88
Range Seven [Member]
Upper range (in shares) | shares	139,484
Weighted average remaining contractual life, outstanding (Year)	6 years 131 days
Weighted average exercise price, outstanding (in dollars per share)	$ 39.86
Exercisable (in shares) | shares	122.392
Weighted average remaining contractual life, exercisable (Year)	6 years 125 days
Weighted average exercise price, exercisable (in dollars per share)	$ 39.86