Note 4 - Stockholders' Equity - Options Outstanding Separated Into Range of Exercise Price (Details)	3 Months Ended
	Mar. 31, 2018 $ / shares shares
Options outstanding (in shares) | shares	1,251,577
Weighted average remaining contractual life (Year)	4 years 227 days
Weighted average exercise price (in dollars per share)	$ 15.935
Options exercisable (in shares) | shares	1,122,468
Weighted average remaining contractual life, exercisable (Year)	4 years 133 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 15.151
Range One [Member]
Lower range of exercise price (in dollars per share)	1.039
Upper range of exercise price (in dollars per share)	$ 1.576
Options outstanding (in shares) | shares	361,725
Weighted average remaining contractual life (Year)	1 year 32 days
Weighted average exercise price (in dollars per share)	$ 1.275
Options exercisable (in shares) | shares	361,725
Weighted average remaining contractual life, exercisable (Year)	1 year 32 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 1.275
Range Two [Member]
Lower range of exercise price (in dollars per share)	6.23
Upper range of exercise price (in dollars per share)	$ 8.80
Options outstanding (in shares) | shares	28,656
Weighted average remaining contractual life (Year)	3 years 283 days
Weighted average exercise price (in dollars per share)	$ 7.174
Options exercisable (in shares) | shares	28,656
Weighted average remaining contractual life, exercisable (Year)	3 years 283 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 7.174
Range Three [Member]
Lower range of exercise price (in dollars per share)	12.47
Upper range of exercise price (in dollars per share)	$ 16.87
Options outstanding (in shares) | shares	204,001
Weighted average remaining contractual life (Year)	5 years 194 days
Weighted average exercise price (in dollars per share)	$ 13.387
Options exercisable (in shares) | shares	180,041
Weighted average remaining contractual life, exercisable (Year)	5 years 80 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 12.923
Range Four [Member]
Lower range of exercise price (in dollars per share)	19.51
Upper range of exercise price (in dollars per share)	$ 21.66
Options outstanding (in shares) | shares	328,024
Weighted average remaining contractual life (Year)	6 years 136 days
Weighted average exercise price (in dollars per share)	$ 21.205
Options exercisable (in shares) | shares	266,979
Weighted average remaining contractual life, exercisable (Year)	6 years 124 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 21.201
Range Five [Member]
Lower range of exercise price (in dollars per share)	22.01
Upper range of exercise price (in dollars per share)	$ 24.23
Options outstanding (in shares) | shares	148,740
Weighted average remaining contractual life (Year)	6 years 17 days
Weighted average exercise price (in dollars per share)	$ 22.297
Options exercisable (in shares) | shares	138,475
Weighted average remaining contractual life, exercisable (Year)	6 years 15 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 22.318
Range Six [Member]
Options outstanding (in shares) | shares	92,028
Weighted average remaining contractual life (Year)	6 years 328 days
Weighted average exercise price (in dollars per share)	$ 29.88
Options exercisable (in shares) | shares	58,189
Weighted average remaining contractual life, exercisable (Year)	6 years 328 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 29.88
Range Seven [Member]
Options outstanding (in shares) | shares	88,403
Weighted average remaining contractual life (Year)	5 years 356 days
Weighted average exercise price (in dollars per share)	$ 39.86
Options exercisable (in shares) | shares	88,403
Weighted average remaining contractual life, exercisable (Year)	5 years 356 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 39.86