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Net Loss Per Share - Antidilutive Securities Excluded from Computation of Earnings Per Share (Details) - $ / shares	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Aug. 31, 2019
Class A Common Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	4,300,000
Shares subject to outstanding common stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	3,639,228	4,957,839
Shares subject to unvested restricted stock awards
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	3,006,717	2,780,345
Shares issuable pursuant to the ESPP
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	103,231	89,311
Convertible debt \| 1.125% Convertible Senior Notes Due 2026
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Conversion price (in dollars per share)			$ 80.16