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, 2017	Dec. 31, 2016	Dec. 31, 2015
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	22,457	30,340	29,656
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	16,714	24,542	24,278
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	1,459	1,631
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	4,284	4,339	3,747