Property and Equipment, net - Summary of Property and Equipment, net (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	$ 21,969	$ 1,400
Accumulated Depreciation	(942)	(72)
Property and equipment, net	21,027	1,328
Computer Equipment and Software [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	110	118
Furniture, Fixtures, and Other [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	2,044	238
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	2,970	861
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	15,780	88
Construction Work in Progress [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment, gross	$ 1,065	$ 95