Common Stock - Schedule of Common Stock Reserved for Future Issuance (Details) - shares	Sep. 30, 2019	Dec. 31, 2018
Class of Stock [Line Items]
Shares of common stock reserved (in shares)	63,320,000	204,278,000
2010 Plan
Class of Stock [Line Items]
Shares of common stock reserved (in shares)	0	13,356,000
2019 Plan
Class of Stock [Line Items]
Shares of common stock reserved (in shares)	29,387,000	0
Options
Class of Stock [Line Items]
Shares of common stock reserved (in shares)	21,878,000	25,087,000
Restricted Stock Units (RSUs)
Class of Stock [Line Items]
Shares of common stock reserved (in shares)	6,185,000	0
ESPP
Class of Stock [Line Items]
Shares of common stock reserved (in shares)	5,870,000	0
Redeemable convertible preferred stock warrants outstanding
Class of Stock [Line Items]
Shares of common stock reserved (in shares)	0	177,000
Redeemable convertible preferred stock
Class of Stock [Line Items]
Shares of common stock reserved (in shares)	0	165,658,000