Redeemable Convertible Preferred Stock and Common Stock - Narrative (Details)			1 Months Ended				12 Months Ended
	Dec. 11, 2018 USD ($) $ / shares shares	Aug. 10, 2016 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) shares	May 31, 2018 USD ($) $ / shares shares	Feb. 28, 2018 USD ($) $ / shares shares	Feb. 15, 2018 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) shares	May 07, 2018 shares
Subsidiary, Sale of Stock [Line Items]
Common stock, shares authorized (in shares)	1,600,000,000				775,000,000		1,600,000,000	1,600,000,000
Redeemable convertible preferred stock, shares authorized	509,352,795								448,686,791	509,352,795
Common stock, par value (usd per share) | $ / shares	$ 0.0001						$ 0.0001	$ 0.0001
Preferred stock, shares authorized (in shares)	162,000,000						162,000,000	162,000,000
Preferred stock, par value (usd per share) | $ / shares	$ 0.0001						$ 0.0001	$ 0.0001
Proceeds from initial public offering of common stock, net of issuance costs | $							$ 0	$ 563,026,000	$ 0
Offering expenses | $								$ 41,322,000
Redeemable convertible preferred stock, shares issued	508,539,515						0	0	448,686,791
Redeemable convertible preferred stock, shares outstanding	508,539,515						0	0	448,686,791
Accretion of redeemable convertible preferred units | $								$ 0
Series F Redeemable Convertible Preferred Stock
Subsidiary, Sale of Stock [Line Items]
Redeemable convertible preferred stock, shares authorized	54,001,241								54,001,241
Shares of common stock issued (in shares)		54,001,241
Price per share (usd per share) | $ / shares		$ 8.78
Aggregate net proceeds from the offering | $		$ 474,100,000
Offering expenses | $		$ 600,000
Redeemable convertible preferred stock, shares issued	54,001,241								54,001,241
Redeemable convertible preferred stock, shares outstanding	54,001,241								54,001,241
Authorized number of shares (in shares)		68,337,129
Conversion price (usd per share) | $ / shares								$ 19.14
Series G Redeemable Convertible Preferred Stock
Subsidiary, Sale of Stock [Line Items]
Redeemable convertible preferred stock, shares authorized	55,666,004
Shares of common stock issued (in shares)					55,666,004	55,666,004
Price per share (usd per share) | $ / shares					$ 10.06	$ 10.06
Aggregate net proceeds from the offering | $					$ 560,000,000	$ 560,000,000
Offering expenses | $					$ 10,500,000		$ 10,517,000	$ 10,517,000
Redeemable convertible preferred stock, shares issued	55,666,004
Redeemable convertible preferred stock, shares outstanding	55,666,004
Conversion price (usd per share) | $ / shares								$ 21.93
Series H Redeemable Convertible Preferred Stock
Subsidiary, Sale of Stock [Line Items]
Redeemable convertible preferred stock, shares authorized	5,000,000
Shares of common stock issued (in shares)				5,000,000
Price per share (usd per share) | $ / shares				$ 25.00
Aggregate net proceeds from the offering | $				$ 112,000,000
Offering expenses | $				$ 400,000			$ 474,000	$ 474,000
Redeemable convertible preferred stock, shares issued	5,000,000
Redeemable convertible preferred stock, shares outstanding	5,000,000
Conversion ratio				0.8771930
Conversion price (usd per share) | $ / shares				$ 21.93				$ 21.93
Fair value per share (usd per share) | $ / shares				$ 22.39
Premium recorded to deferred revenue | $				$ 13,000,000
Convertible holding period								21 months
Percentage discount on shares								10.00%
Series D Redeemable Convertible Preferred Stock
Subsidiary, Sale of Stock [Line Items]
Redeemable convertible preferred stock, shares authorized	63,291,156								63,291,156
Redeemable convertible preferred stock, shares issued	63,021,976								63,291,156
Redeemable convertible preferred stock, shares outstanding	63,021,976								63,291,156
Conversion price (usd per share) | $ / shares								$ 4.65
Repurchase of redeemable convertible preferred stock (in shares)			269,180
Repurchase of redeemable convertible preferred stock | $								$ 2,009,000
Dividend rate per annum (usd per share | $ / shares								$ 0.171
Series E Redeemable Convertible Preferred Stock
Subsidiary, Sale of Stock [Line Items]
Redeemable convertible preferred stock, shares authorized	81,428,340								81,428,340
Redeemable convertible preferred stock, shares issued	80,884,240								81,428,340
Redeemable convertible preferred stock, shares outstanding	80,884,240								81,428,340
Conversion price (usd per share) | $ / shares								$ 13.45
Repurchase of redeemable convertible preferred stock (in shares)			544,100
Repurchase of redeemable convertible preferred stock | $			$ 8,200,000					$ 2,118,000
Series A Redeemable Convertible Preferred Stock
Subsidiary, Sale of Stock [Line Items]
Redeemable convertible preferred stock, shares authorized	42,000,000								42,000,000
Redeemable convertible preferred stock, shares issued	42,000,000								42,000,000
Redeemable convertible preferred stock, shares outstanding	42,000,000								42,000,000
Conversion price (usd per share) | $ / shares								$ 0.11
Dividend rate per annum (usd per share | $ / shares								0.004
Series B Redeemable Convertible Preferred Stock
Subsidiary, Sale of Stock [Line Items]
Redeemable convertible preferred stock, shares authorized	122,296,280								122,296,280
Redeemable convertible preferred stock, shares issued	122,296,280								122,296,280
Redeemable convertible preferred stock, shares outstanding	122,296,280								122,296,280
Conversion price (usd per share) | $ / shares								0.17
Dividend rate per annum (usd per share | $ / shares								0.006
Series C Redeemable Convertible Preferred Stock
Subsidiary, Sale of Stock [Line Items]
Redeemable convertible preferred stock, shares authorized	85,669,774								85,669,774
Redeemable convertible preferred stock, shares issued	85,669,774								85,669,774
Redeemable convertible preferred stock, shares outstanding	85,669,774								85,669,774
Conversion price (usd per share) | $ / shares								0.70
Dividend rate per annum (usd per share | $ / shares								$ 0.02568
IPO
Subsidiary, Sale of Stock [Line Items]
Shares of common stock issued (in shares)	26,275,993
Price per share (usd per share) | $ / shares	$ 23.00
Aggregate net proceeds from the offering | $	$ 563,000,000
Underwriting discounts | $	33,200,000
Offering expenses | $	$ 8,100,000
IPO | Common Stock
Subsidiary, Sale of Stock [Line Items]
Preferred stock converted into common stock (in shares)	236,012,913