Significant Accounting Policies - Schedule of Estimated Useful Lives of Asset Classification (Detail)	12 Months Ended
	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Leasehold improvements	Shorter of useful life or remaining life of lease
Computer Hardware and Software [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Estimated Useful Life	3 years
Computer Hardware and Software [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Estimated Useful Life	5 years
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Estimated Useful Life	5 years
Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, Estimated Useful Life	5 years