Note 7 - Summary of Stock Option Activity (Details) - USD ($)		6 Months Ended	12 Months Ended
		Jun. 30, 2016	Dec. 31, 2015
Options outstanding, beginning balance (in shares)		7,198,479
Weighted average exercise price, beginning balance (in dollars per share)		$ 3.97
Weighted average remaining contractual life, beginning balance		7 years 73 days	7 years
Aggregate intrinsic value	[1]	$ 31,185,000	$ 22,875,000
Options outstanding, granted (in shares)		872,017
Weighted average exercise price, granted (in dollars per share)		$ 8.49
Options outstanding, exercised (in shares)		(783,042)
Weighted average exercise price, exercised (in dollars per share)		$ 3.21
Options outstanding, forfeited (in shares)		(53,316)
Weighted average exercise price, forfeited (in dollars per share)		$ 5.44
Options outstanding, ending balance (in shares)		7,234,138	7,198,479
Weighted average exercise price, ending balance (in dollars per share)		$ 4.58	$ 3.97
Options outstanding, vested and exercisable (in shares)		4,439,757
Weighted average exercise price, vested and exercisable (in dollars per share)		$ 4.20
Weighted average remaining contractual life, vested and exercisable		6 years 36 days
Aggregate intrinsic value, vested and exercisable	[1]	$ 20,838,000
Options outstanding, vested and exercisable and expected to vest thereafter (in shares)	[2]	6,780,263
Weighted average exercise price, vested and exercisable and expected to vest thereafter (in dollars per share)	[2]	$ 4.53
Weighted average remaining contractual life, vested and exercisable and expected to vest thereafter	[2]	7 years
Aggregate intrinsic value, vested and exercisable and expected to vest thereafter	[1],[2]	$ 29,888,000

[1] The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of our common stock as of June 30, 2016 of $8.89 per share

[2] Options that are expected to vest are net of estimated future option forfeitures in accordance with the provisions of ASC 718, “Compensation — Stock Compensation.”