Nature of the Business - Additional Information (Detail) - USD ($) $ in Thousands	105 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Nature Of Business [Line Items]
Accumulated deficit	$ (963,329)	$ (590,447)
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	$ 1,600