Stock-Based Compensation (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		1 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Jan. 31, 2015	Dec. 31, 2014
Common stock options
Shares
Outstanding at the beginning of the period (in shares)	8,829,343		8,829,343
Options granted (in shares)	1,693,686
Options cancelled (in shares)	(399,825)
Outstanding at the end of the period (in shares)	10,123,204
Exercisable at the end of the period (in shares)	5,305,790
Weighted average exercise price
Outstanding at the beginning of the period (in dollars per share)	$ 6.17		6.17
Options granted (in dollars per share)	$ 2.39
Options cancelled (in dollars per share)	$ 9.14
Outstanding at the end of the period (in dollars per share)	$ 5.42
Exercisable at the end of the period (in dollars per share)	$ 6.45
Additional disclosures
Total cash received from stock option exercises	$ 0	$ 0.8
Weighted-average grant date fair value of options (in dollars per share)	$ 1.92	$ 5.02
Weighted average assumptions used to estimate fair value of each employee stock option award
Risk-free interest rate (as a percent)	1.77%	1.84%
Expected life in years	6 years 3 months	6 years 3 months
Volatility (as a percent)	101.00%	104.00%
Restricted stock
Additional disclosures
Total fair value of restricted stock vested during the period	$ 0.1	$ 0.1
Shares
Outstanding at the beginning of the period (in shares)	744,514		744,514
Vested (in shares)	(9,756)
Outstanding at the end of the period (in shares)	734,758
Weighted average grant date fair value
Outstanding at the beginning of the period (in dollars per share)	$ 3.65		3.65
Vested (in dollars per share)	$ 4.10
Outstanding at the end of the period (in dollars per share)	$ 3.65
2006 Stock Plan
Stock-based compensation expense
Shares reserved for issuance			11,600,000	10,300,000
Increase in number of shares reserved for issuance			1,300,000
Percentage of outstanding shares of common stock used to compute annual increase in number of shares reserved for issuance			5.00%
Shares available for future issuance	1,261,136
2006 Stock Plan | Common stock options | Maximum
Stock-based compensation expense
Expiration period	10 years
Vesting period	4 years