Note 4 - Options Outstanding Separated into Range of Exercise Price (Details) - $ / shares	3 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Range One [Member]
Lower range (in dollars per share)	$ 0.901
Upper Range (in dollars per share)	$ 1.576
Options outstanding as of March 31, 2016 (in shares)	974,178
Weighted average remaining contractual life	2 years 248 days
Weighted average exercise price (in dollars per share)	$ 1.225
Options exercisable as of March 31, 2016 (in shares)	974,178
Weighted average remaining contractual life	2 years 248 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 1.225
Range Two [Member]
Lower range (in dollars per share)	6.23
Upper Range (in dollars per share)	$ 8.80
Options outstanding as of March 31, 2016 (in shares)	136,650
Weighted average remaining contractual life	5 years 244 days
Weighted average exercise price (in dollars per share)	$ 6.806
Options exercisable as of March 31, 2016 (in shares)	133,289
Weighted average remaining contractual life	5 years 240 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 6.807
Range Three [Member]
Lower range (in dollars per share)	12.47
Upper Range (in dollars per share)	$ 16.87
Options outstanding as of March 31, 2016 (in shares)	510,698
Weighted average remaining contractual life	7 years 259 days
Weighted average exercise price (in dollars per share)	$ 13.633
Options exercisable as of March 31, 2016 (in shares)	281,231
Weighted average remaining contractual life	6 years 332 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 12.47
Range Four [Member]
Lower range (in dollars per share)	19.51
Upper Range (in dollars per share)	$ 21.66
Options outstanding as of March 31, 2016 (in shares)	622,311
Weighted average remaining contractual life	8 years 132 days
Weighted average exercise price (in dollars per share)	$ 21.218
Options exercisable as of March 31, 2016 (in shares)	247,216
Weighted average remaining contractual life	8 years 90 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 21.243
Range Five [Member]
Lower range (in dollars per share)	22.01
Upper Range (in dollars per share)	$ 24.23
Options outstanding as of March 31, 2016 (in shares)	308,524
Weighted average remaining contractual life	8 years 18 days
Weighted average exercise price (in dollars per share)	$ 22.289
Options exercisable as of March 31, 2016 (in shares)	145,959
Weighted average remaining contractual life	8 years 12 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 22.349
Range Six [Member]
Options outstanding as of March 31, 2016 (in shares)	154,200
Weighted average remaining contractual life	8 years 328 days
Weighted average exercise price (in dollars per share)	$ 29.88
Options exercisable as of March 31, 2016 (in shares)	41,772
Weighted average remaining contractual life	8 years 328 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 29.88
Range Seven [Member]
Options outstanding as of March 31, 2016 (in shares)	172,427
Weighted average remaining contractual life	7 years 356 days
Weighted average exercise price (in dollars per share)	$ 39.86
Options exercisable as of March 31, 2016 (in shares)	87,852
Weighted average remaining contractual life	7 years 356 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 39.86
Options outstanding as of March 31, 2016 (in shares)	2,878,988
Weighted average remaining contractual life	6 years 61 days
Weighted average exercise price (in dollars per share)	$ 14.119	$ 14.026
Options exercisable as of March 31, 2016 (in shares)	1,911,497
Weighted average remaining contractual life	5 years 6 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 9.873