Nature of the Business - Additional Information (Detail) - USD ($) $ in Thousands						93 Months Ended
	Feb. 13, 2018	Nov. 17, 2017	Sep. 14, 2016	Jan. 12, 2016	Apr. 20, 2015	Dec. 31, 2017	Dec. 31, 2016
Nature Of Business [Line Items]
Accumulated deficit						$ (590,447)	$ (320,327)
Net proceeds from sale of common stock		$ 325,800	$ 189,200	$ 140,400	$ 129,100
Subsequent Event [Member]
Nature Of Business [Line Items]
Net proceeds from sale of common stock	$ 631,100
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						$ 969,000