Note 4 - Stockholders' Equity - Options Outstanding Separated Into Range of Exercise Price (Details)	6 Months Ended
	Jun. 30, 2017 $ / shares shares
Weighted average exercise price, outstanding (in dollars per share)	$ 15.941
Exercisable (in shares) | shares	1,633,271
Weighted average remaining contractual life, exercisable (Year)	4 years 227 days
Weighted average exercise price, exercisable (in dollars per share)	$ 14.216
Outstanding (in shares) | shares	1,985,104
Weighted average remaining contractual life, outstanding (Year)	5 years 12 days
Range One [Member]
Weighted average exercise price, outstanding (in dollars per share)	$ 1.329
Exercisable (in shares) | shares	575,859
Weighted average remaining contractual life, exercisable (Year)	1 year 312 days
Weighted average exercise price, exercisable (in dollars per share)	$ 1.329
Lower range (in dollars per share)	1.039
Upper range (in dollars per share)	$ 1.576
Outstanding (in shares) | shares	575,859
Weighted average remaining contractual life, outstanding (Year)	1 year 312 days
Range Two [Member]
Weighted average exercise price, outstanding (in dollars per share)	$ 7.032
Exercisable (in shares) | shares	70,930
Weighted average remaining contractual life, exercisable (Year)	4 years 97 days
Weighted average exercise price, exercisable (in dollars per share)	$ 7.032
Lower range (in dollars per share)	6.23
Upper range (in dollars per share)	$ 8.80
Outstanding (in shares) | shares	70,930
Weighted average remaining contractual life, outstanding (Year)	4 years 97 days
Range Three [Member]
Weighted average exercise price, outstanding (in dollars per share)	$ 13.429
Exercisable (in shares) | shares	263,780
Weighted average remaining contractual life, exercisable (Year)	5 years 313 days
Weighted average exercise price, exercisable (in dollars per share)	$ 12.995
Lower range (in dollars per share)	12.47
Upper range (in dollars per share)	$ 16.87
Outstanding (in shares) | shares	297,114
Weighted average remaining contractual life, outstanding (Year)	6 years 62 days
Range Four [Member]
Weighted average exercise price, outstanding (in dollars per share)	$ 21.202
Exercisable (in shares) | shares	311,878
Weighted average remaining contractual life, exercisable (Year)	6 years 186 days
Weighted average exercise price, exercisable (in dollars per share)	$ 21.20
Lower range (in dollars per share)	19.51
Upper range (in dollars per share)	$ 21.66
Outstanding (in shares) | shares	475,348
Weighted average remaining contractual life, outstanding (Year)	6 years 219 days
Range Five [Member]
Weighted average exercise price, outstanding (in dollars per share)	$ 22.27
Exercisable (in shares) | shares	204,808
Weighted average remaining contractual life, exercisable (Year)	6 years 115 days
Weighted average exercise price, exercisable (in dollars per share)	$ 22.305
Lower range (in dollars per share)	22.01
Upper range (in dollars per share)	$ 24.23
Outstanding (in shares) | shares	269,316
Weighted average remaining contractual life, outstanding (Year)	6 years 122 days
Range Six [Member]
Weighted average exercise price, outstanding (in dollars per share)	$ 29.88
Exercisable (in shares) | shares	86,643
Weighted average remaining contractual life, exercisable (Year)	7 years 236 days
Weighted average exercise price, exercisable (in dollars per share)	$ 29.88
Outstanding (in shares) | shares	150,887
Weighted average remaining contractual life, outstanding (Year)	7 years 236 days
Range Seven [Member]
Weighted average exercise price, outstanding (in dollars per share)	$ 39.86
Exercisable (in shares) | shares	119,373
Weighted average remaining contractual life, exercisable (Year)	5 years 158 days
Weighted average exercise price, exercisable (in dollars per share)	$ 39.86
Outstanding (in shares) | shares	145,650
Weighted average remaining contractual life, outstanding (Year)	5 years 158 days