Segment and Geographic Information - Summary of Reconciliation of Segment Financial Information to Loss from Operations (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Research and development expenses	$ 313,915	$ 75,880	$ 466,530	$ 229,728
General and administrative expenses	338,977	74,062	503,033	208,736
Stock-based compensation expense	846,959	51,763	1,028,914	164,650
Loss from operations	(847,777)	(144,140)	(1,017,107)	(428,993)
Reconciling items
Segment Reporting Information [Line Items]
Research and development expenses	57,146	60,849	156,832	180,591
General and administrative expenses	107,130	60,411	226,455	165,985
Stock-based compensation expense	846,959	51,763	1,028,914	164,650
Operating Segments
Segment Reporting Information [Line Items]
Loss from operations	$ 163,458	$ 28,883	$ 395,094	$ 82,233

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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