Note 6 - Stock-based Compensation (Tables)

3 Months Ended

Mar. 31, 2016

Notes Tables

Schedule of Share-based Compensation, Stock Options, Activity [Table Text Block]

	Options			Weighted-Average Exercise Price		Weighted-Average Remaining Contractual Term (years)		Aggregate Intrinsic Value
Outstanding at December 31, 2015		2,200,369		$	8.52		7.8	$	15,705
Granted		10,000		$	6.68
Exercised		(6,394	)	$	1.92
Outstanding at March 31, 2016		2,203,975		$	8.53		7.6	$	4,445
Exercisable at March 31, 2016		939,661		$	4.35		5.6	$	3,657
Vested or expected to vest at March 31, 2016 (1)		2,021,309		$	8.22		7.4	$	4,355

Common Stock Reserved for Future Issuance [Table Text Block]

	March 31,		December 31,
	2016		2015
Stock-based compensation awards		3,073,567		2,419,901
Employee Stock Purchase Plan		297,359		297,359
Total		3,370,926		2,717,260