Summary of Siginificant Accounting Policies - Stock-Based Compensation, Per Share Amounts (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended			3 Months Ended								12 Months Ended
	Jan. 31, 2014	Feb. 28, 2013	May. 31, 2011	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	May. 31, 2013
Stock-Based Compensation
Tax benefit (expense) for stock-based compensation												$ 0	$ 0	$ 0
Numerator
Net loss attributable to common shareholders				$ (25,227)	$ (10,238)	$ (9,253)	$ (11,077)	$ (7,183)	$ (9,377)	$ 3,825	$ (75,909)	$ (55,795)	$ (88,644)	$ (62,791)
Denominator
Weighted Average Number of Shares Outstanding, Basic and Diluted												176,067,231	159,228,815	76,436,408
Number of dilutive potential common shares												21,652,391	18,614,650	40,464,065
Number of warrants exercised (in shares)												26,882	23,918,429	22,494,987
Weighted average exercise price of warrants (in dollars per share)								$ 4.00					$ 4.00
Series C redeemable convertible Preferred Stock, shares issued				5,231				5,231				5,231	5,231
Minimum
Denominator
Weighted average exercise price of warrants (in dollars per share)														$ 0.15
Maximum
Denominator
Weighted average exercise price of warrants (in dollars per share)														$ 0.18
Stock options
Denominator
Number of dilutive potential common shares												11,700,786	8,367,271	4,703,326
Restricted stock
Denominator
Number of dilutive potential common shares												204,444	473,336	650,002
Warrants
Denominator
Number of dilutive potential common shares												4,192,567	4,219,449	24,137,878
Preferred stock
Denominator
Number of dilutive potential common shares												5,554,594	5,554,594	10,972,859
Warrants Issued Underwitten Public Offerings
Denominator
Number of warrants issued (in shares)			7,128,563
Warrants issued in May, 2011
Denominator
Number of warrants issued (in shares)			22,995,365
Number of warrants exercised (in shares)												22,802,898
Warrants issued in February, 2013
Denominator
Number of warrants issued (in shares)		23,637,500
Number of warrants exercised (in shares)												23,637,400
Warrants issued in January, 2014
Denominator
Number of warrants issued (in shares)	4,000,000
Number of warrants exercised (in shares)												0
Series C redeemable convertible preferred stock
Denominator
Series C redeemable convertible Preferred Stock, shares issued															10,431
Shares issued upon conversion of redeemable stock (in shares)												5,200