Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands				9 Months Ended
	May 21, 2020	Feb. 14, 2020	Dec. 11, 2018	Sep. 30, 2020	Sep. 30, 2019	[1]	Dec. 31, 2019	May 07, 2018	Feb. 28, 2018
Subsidiary, Sale of Stock [Line Items]
Common stock, shares authorized (in shares)			1,600,000,000	1,600,000,000			1,600,000,000		775,000,000
Redeemable convertible preferred stock, shares authorized (in shares)								509,352,795
Common stock, par value (usd per share)			$ 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)			$ 0.0001	$ 0.0001			$ 0.0001
Offering expenses				$ 2,086
Proceeds from public offerings of common stock, net of issuance costs				$ 1,852,759	$ 0
IPO
Subsidiary, Sale of Stock [Line Items]
Shares of common stock issued (in shares)			26,275,993
Price per share (usd per share)			$ 23.00
Aggregate net proceeds from the offering			$ 563,000
Underwriting discounts			33,200
Offering expenses			$ 8,100
IPO | Common Stock
Subsidiary, Sale of Stock [Line Items]
Preferred stock converted into common stock (in shares)			236,012,913
Public equity offering
Subsidiary, Sale of Stock [Line Items]
Shares of common stock issued (in shares)	17,600,000	26,315,790
Price per share (usd per share)	$ 76.00	$ 19.00
Proceeds from public offerings of common stock, net of issuance costs	$ 1,300,000	$ 477,700
Underwriting option
Subsidiary, Sale of Stock [Line Items]
Shares of common stock issued (in shares)		3,947,368
Proceeds from public offerings of common stock, net of issuance costs		$ 71,800

[1] Restated to conform to ASC 842. See accompanying Note 2.