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, 2019	Dec. 31, 2018	Dec. 31, 2017
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	17,976	20,950	22,457
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	10,626	14,115	16,714
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	1,459
Unvested restricted stock units
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	5,891	5,376	4,284