Note 4 - Options Outstanding Separated into Range of Exercise Price (Details) - $ / shares	6 Months Ended
	Jun. 30, 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 (in shares)	900,240
Weighted average remaining contractual life	2 years 205 days
Weighted average exercise price (in dollars per share)	$ 1.247
Options exercisable (in shares)	900,240
Weighted average remaining contractual life	2 years 205 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 1.247
Range Two [Member]
Lower range (in dollars per share)	6.23
Upper range (in dollars per share)	$ 8.80
Options outstanding (in shares)	116,243
Weighted average remaining contractual life	5 years 141 days
Weighted average exercise price (in dollars per share)	$ 6.834
Options exercisable (in shares)	116,243
Weighted average remaining contractual life	5 years 141 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 6.834
Range Three [Member]
Lower range (in dollars per share)	12.47
Upper range (in dollars per share)	$ 16.87
Options outstanding (in shares)	492,805
Weighted average remaining contractual life	7 years 178 days
Weighted average exercise price (in dollars per share)	$ 13.675
Options exercisable (in shares)	286,177
Weighted average remaining contractual life	6 years 241 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 (in shares)	583,615
Weighted average remaining contractual life	8 years 42 days
Weighted average exercise price (in dollars per share)	$ 21.208
Options exercisable (in shares)	264,672
Weighted average remaining contractual life	8 years 12 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 21.203
Range Five [Member]
Lower range (in dollars per share)	22.01
Upper range (in dollars per share)	$ 24.23
Options outstanding (in shares)	307,181
Weighted average remaining contractual life	7 years 292 days
Weighted average exercise price (in dollars per share)	$ 22.291
Options exercisable (in shares)	164,615
Weighted average remaining contractual life	7 years 286 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 22.343
Range Six [Member]
Options outstanding (in shares)	154,200
Weighted average remaining contractual life	8 years 236 days
Weighted average exercise price (in dollars per share)	$ 29.88
Options exercisable (in shares)	51,406
Weighted average remaining contractual life	8 years 236 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 29.88
Range Seven [Member]
Options outstanding (in shares)	164,641
Weighted average remaining contractual life	7 years 266 days
Weighted average exercise price (in dollars per share)	$ 39.86
Options exercisable (in shares)	96,019
Weighted average remaining contractual life	7 years 266 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 39.86
Options outstanding (in shares)	2,718,925
Weighted average remaining contractual life	6 years 6 days
Weighted average exercise price (in dollars per share)	$ 14.362	$ 14.026
Options exercisable (in shares)	1,879,372
Weighted average remaining contractual life	5 years 7 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 10.716