Note 6 - Stock-based Compensation (Tables)

6 Months Ended

Jun. 30, 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		114,162		$	6.77
Exercised		(72,508	)	$	1.91
Cancelled or forfeited		(7,089	)	$	4.92
Outstanding at June 30, 2016		2,234,934		$	8.66		7.6	$	4,470
Exercisable at June 30, 2016		997,718		$	5.57		5.9	$	3,577
Vested or expected to vest at June 30, 2016 (1)		2,064,955		$	8.41		7.4	$	4,373

Common Stock Reserved for Future Issuance [Table Text Block]

	June 30, 2016		December 31, 2015
Stock-based compensation awards		3,007,453		2,419,901
Employee Stock Purchase Plan		296,685		297,359
Total		3,304,138		2,717,260