Nature of the Business - Additional Information (Detail) - USD ($) $ in Thousands	111 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Nature Of Business [Line Items]
Accumulated deficit	$ (1,294,956)	$ (963,329)
Redeemable Convertible Preferred Stock [Member] | Convertible Notes [Member] | Initial Public Offering [Member]
Nature Of Business [Line Items]
Net proceeds from sale of equity and notes	$ 2,200,000