Income Taxes - Summary of Significant Components of Company's Net Deferred Tax Asset (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Net operating losses	$ 348,848	$ 206,460
Capitalized start-up costs	982	1,242
Tax credit carryforwards	80,088	63,653
Accrued expenses	4,055	4,498
Depreciation and amortization	2,002	1,001
Stock options	46,750	30,925
Right of use asset	(8,059)
Right of use liability	8,852
Other	(246)	568
Total net deferred tax asset before valuation allowance	483,272	308,347
Valuation allowance	$ (483,272)	$ (308,347)