Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2014

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

Summary of the activity related to the shares of common stock covered by outstanding options under the plans

A summary of the activity related to the shares of common stock covered by outstanding options is as follows:

	Number of Shares		Weighted Average Exercise Price	Remaining Contractual Term (in years)	Aggregate Intrinsic Value
Balance at December 31, 2013	6,201,429		$5.68
Granted	2,051,925		1.27
Exercised	(250,000	)	1.20
Forfeited	(722,487	)	2.39
Expired	(616,508	)	7.27
Balance at December 31, 2014	6,664,359		4.70	5.75	$—
Vested and expected to vest at December 31, 2014	6,426,083		4.82	5.64	—
Exercisable at December 31, 2014	4,416,231		6.30	4.28	—

Summary of information about the shares of common stock covered by outstanding and exercisable options under the plans

A summary of information about the shares of common stock covered by outstanding and exercisable options at December 31, 2014 follows:

	Stock Options Outstanding				Stock Options Exercisable
Range of exercise prices	Number of shares	Weighted average remaining contractual life (in years)	Weighted average exercise price per share		Number of shares	Weighted average exercise price per share
$0.55 - 1.29	627,796	8.85	$	1.19	197,338	$	1.24
1.33 - 1.33	1,150,000	8.97	1.33		—	—
1.34-1.64	1,108,193	5.58	1.54		983,855	1.55
1.65 - 2.66	1,622,301	5.68	2.15		1,124,512	2.19
2.72 - 9.77	1,115,402	3.73	7.54		1,069,859	7.60
10.08 - 24.97	1,040,667	2.81	14.85		1,040,667	14.85
	6,664,359	5.75	$	4.70	4,416,231	$	6.30

Schedule of assumptions used in determining fair value of stock options granted using the Black-Scholes option pricing model

For the years ended December 31, 2014, 2013 and 2012, the Company determined the fair value of stock options using the Black-Scholes option pricing model with the following assumptions for option grants, respectively:

	Year Ended December 31,
	2014	2013	2012
Expected dividend yield	—	—	—
Risk-free rate	0.01% - 2.41%	0.71% - 2.05%	0.67% - 1.15%
Expected option term (in years)	0.1-6.1	6.0 - 6.1	5.3 - 5.5
Volatility	84% - 85%	84% - 85%	84% - 87%