Net Loss per Share - Potentially Dilutive Shares (Details) - shares shares in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive shares not included in diluted per share calculation	17,548	18,699
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 calculation	6,012	7,265
Restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive shares not included in diluted per share calculation	3,782	3,901
Employee stock purchase plan
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive shares not included in diluted per share calculation	73	122
Convertible Notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive shares not included in diluted per share calculation	7,411	7,411
Estimated earnout shares for an asset acquisition
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive shares not included in diluted per share calculation	270