Income Taxes - Summary of Significant Components of Company's Net Deferred Tax Asset (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Net operating losses	$ 206,460	$ 131,151
Capitalized start-up costs	1,242	1,362
Tax credit carryforwards	63,653	54,523
Accrued expenses	4,498	3,057
Depreciation and amortization	1,001	685
Stock options	30,925	13,573
Others	568	1,090
Total net deferred tax asset before valuation allowance	308,347	205,441
Valuation allowance	$ (308,347)	$ (205,441)