Net Loss Per Share - Schedule of Anti-dilutive Shares Excluded from the Calculation of Diluted Net Loss Per Share (Details 2) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Anti-dilutive shares excluded from the calculation of diluted net loss per share attributable to common stockholders
Total shares excluded from net loss per share attributable to common stockholders	30,340	29,656	30,344
Options to purchase common stock
Anti-dilutive shares excluded from the calculation of diluted net loss per share attributable to common stockholders
Total shares excluded from net loss per share attributable to common stockholders	24,542	24,278	25,590
Common stock warrants
Anti-dilutive shares excluded from the calculation of diluted net loss per share attributable to common stockholders
Total shares excluded from net loss per share attributable to common stockholders	1,459	1,631	3,926
Unvested restricted stock awards
Anti-dilutive shares excluded from the calculation of diluted net loss per share attributable to common stockholders
Total shares excluded from net loss per share attributable to common stockholders	4,339	3,747	828