Note 6 - Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2014

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table Text Block]

	Year Ended December 31,
	2014		2013		2012
Research and development	$	8,906	$	3,664	$	3,511
General and administrative		1,271		704		656
Total stock-based compensation expense	$	10,177	$	4,368	$	4,167

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

	Awards Available			Options Outstanding			Weighted Average Exercise Price
Balance at December 31, 2011		2,009			7,858		$	6.21
Plan amendment		1,700			—			—
Restricted stock awards granted		(415	)		—			—
Restricted stock awards cancelled		86			—			—
Stock option awards granted		(1,617	)		1,617			4.65
Stock option awards exercised		—			(350	)		1.58
Stock option awards cancelled		1,052			(1,052	)		6.26
Balance at December 31, 2012		2,815			8,073			6.09
Restricted stock awards granted		(310	)		—			—
Restricted stock awards cancelled		53			—			—
Stock option awards granted		(3,277	)		3,277			3.05
Stock option awards exercised		—			(563	)		2.37
Stock option awards cancelled		1,801			(1,801	)		7.22
Balance at December 31, 2013		1,082			8,986			4.99
Plan amendment		3,750			—			—
Restricted stock awards granted		(593	)		—			—
Restricted stock awards cancelled		—			—			—
Stock option awards granted		(1,965	)		1,965			10.99
Stock option awards exercised		—			(1,258	)		4.78
Stock option awards cancelled		88			(88	)		8.83
Balance at December 31, 2014		2,362			9,605		$	6.21

Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]

	2014			2013			2012
Expected Life		5.5			4.7			5.4
Expected Volatility		87	%		84	%		87	%
Expected Dividend Yield		0.0	%		0.0	%		0.0	%
Risk-Free Interest Rate		1.6	%		0.7	%		0.9	%

Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding and Exercisable [Table Text Block]

				Outstanding				Exercisable
Range				Number	Weighted Average Remaining Life		Weighted Average Exercise Price	Number		Weighted Average Exercise Price
$	0	to	3	2,236	7.2	$	1.52	1,145	$	1.59
	3	to	6	3,214	6.8		4.59	1,955		4.35
	6	to	9	1,380	5.2		7.12	1,197		7.11
	9	to	12	2,315	7.9		11.16	554		11.62
	12	to	15	456	3.2		12.69	365		12.53
	15	to	18	4	1.0		15.45	4		15.45
$	0	to	18	9,605	6.8	$	6.21	5,220	$	5.73

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

	Non-Vested Stock Option Awards			Weighted Average Grant-Date Fair Value
Balance December 31, 2013		4,209		$	1.50
Stock option awards granted		1,965			7.76
Stock option awards vested		(1,789	)		1.59
Stock option awards forfeited		—			—
Balance December 31, 2014		4,385		$	4.27