Net Loss Per Share - Potentially Dilutive Securities Excluded from the Computation of Diluted Net Loss Per Share (Details) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from the computation of diluted net loss per share (in shares)	20,050	22,755	61,865
Redeemable convertible preferred stock (on an if-converted basis)
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from the computation of diluted net loss per share (in shares)	0	0	41,628
Stock-options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from the computation of diluted net loss per share (in shares)	15,684	22,013	18,701
Redeemable convertible preferred stock warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from the computation of diluted net loss per share (in shares)	0	0	618
Restricted stock and restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from the computation of diluted net loss per share (in shares)	4,347	686	803
Early exercised options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from the computation of diluted net loss per share (in shares)	19	56	115