Summary of significant accounting policies and basis of presentation - Estimated Useful Lives of Assets (Detail)	12 Months Ended
	Dec. 31, 2016
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Estimated useful lives of assets	3 years
Office and Laboratory Equipment [Member] | Minimum [Member]
Property Plant And Equipment [Line Items]
Estimated useful lives of assets	2 years
Office and Laboratory Equipment [Member] | Maximum [Member]
Property Plant And Equipment [Line Items]
Estimated useful lives of assets	5 years
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Leasehold improvements	Shorter of the useful life or remaining lease term