Nature of Operations - Additional Information (Detail) - USD ($) $ in Thousands	78 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Nature Of Operations [Line Items]
Accumulated deficit	$ (264,426)	$ (161,340)
Cash, cash equivalents and marketable securities	431,270
Redeemable Convertible Preferred Stock [Member] | Convertible Notes [Member] | Initial Public Offering [Member]
Nature Of Operations [Line Items]
Net proceeds from sale of equity and notes	$ 643,300