Note 4 - Stockholders' Equity - Options Outstanding Separated Into Range of Exercise Price (Details)	3 Months Ended
	Mar. 31, 2017 $ / shares shares
Outstanding (in shares) | shares	2,256,529
Weighted average remaining contractual life, outstanding (Year)	5 years 251 days
Weighted average exercise price, outstanding (in dollars per share)	$ 15.654
Exercisable (in shares) | shares	1,750,759
Weighted average remaining contractual life, exercisable (Year)	5 years 41 days
Weighted average exercise price, exercisable (in dollars per share)	$ 13.536
Range One [Member]
Lower range (in dollars per share)	1.039
Upper range (in dollars per share)	$ 1.576
Outstanding (in shares) | shares	640,966
Weighted average remaining contractual life, outstanding (Year)	2 years 66 days
Weighted average exercise price, outstanding (in dollars per share)	$ 1.323
Exercisable (in shares) | shares	640,966
Weighted average remaining contractual life, exercisable (Year)	2 years 66 days
Weighted average exercise price, exercisable (in dollars per share)	$ 1.323
Range Two [Member]
Lower range (in dollars per share)	6.23
Upper range (in dollars per share)	$ 8.80
Outstanding (in shares) | shares	82,174
Weighted average remaining contractual life, outstanding (Year)	4 years 267 days
Weighted average exercise price, outstanding (in dollars per share)	$ 6.991
Exercisable (in shares) | shares	82,174
Weighted average remaining contractual life, exercisable (Year)	4 years 267 days
Weighted average exercise price, exercisable (in dollars per share)	$ 6.991
Range Three [Member]
Lower range (in dollars per share)	12.47
Upper range (in dollars per share)	$ 16.87
Outstanding (in shares) | shares	413,768
Weighted average remaining contractual life, outstanding (Year)	6 years 327 days
Weighted average exercise price, outstanding (in dollars per share)	$ 13.906
Exercisable (in shares) | shares	305,515
Weighted average remaining contractual life, exercisable (Year)	6 years 102 days
Weighted average exercise price, exercisable (in dollars per share)	$ 12.997
Range Four [Member]
Lower range (in dollars per share)	19.51
Upper range (in dollars per share)	$ 21.66
Outstanding (in shares) | shares	529,192
Weighted average remaining contractual life, outstanding (Year)	7 years 143 days
Weighted average exercise price, outstanding (in dollars per share)	$ 21.205
Exercisable (in shares) | shares	326,929
Weighted average remaining contractual life, exercisable (Year)	7 years 124 days
Weighted average exercise price, exercisable (in dollars per share)	$ 21.202
Range Five [Member]
Lower range (in dollars per share)	22.01
Upper range (in dollars per share)	$ 24.23
Outstanding (in shares) | shares	288,474
Weighted average remaining contractual life, outstanding (Year)	7 years 18 days
Weighted average exercise price, outstanding (in dollars per share)	$ 22.287
Exercisable (in shares) | shares	203,947
Weighted average remaining contractual life, exercisable (Year)	7 years 14 days
Weighted average exercise price, exercisable (in dollars per share)	$ 22.327
Range Six [Member]
Outstanding (in shares) | shares	154,200
Weighted average remaining contractual life, outstanding (Year)	7 years 328 days
Weighted average exercise price, outstanding (in dollars per share)	$ 29.88
Exercisable (in shares) | shares	80,322
Weighted average remaining contractual life, exercisable (Year)	7 years 328 days
Weighted average exercise price, exercisable (in dollars per share)	$ 29.88
Range Seven [Member]
Outstanding (in shares) | shares	147,755
Weighted average remaining contractual life, outstanding (Year)	6 years 356 days
Weighted average exercise price, outstanding (in dollars per share)	$ 39.86
Exercisable (in shares) | shares	110,906
Weighted average remaining contractual life, exercisable (Year)	6 years 356 days
Weighted average exercise price, exercisable (in dollars per share)	$ 39.86