Net Loss per Share - Potentially Dilutive Shares (Details) - shares shares in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive shares not included in diluted per share calculations	12,304	9,119
Employee and non-employee stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive shares not included in diluted per share calculations	10,732	8,774
Warrants to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive shares not included in diluted per share calculations	377
Restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive shares not included in diluted per share calculations	1,014	138
Employee stock purchase plan
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive shares not included in diluted per share calculations	181	207