Summary of Significant Accounting Policies and Basis of Presentation - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2016 USD ($) Segment Award	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Oct. 31, 2016 USD ($)	Mar. 31, 2016	Feb. 12, 2016
Summary Of Significant Accounting Policies [Line Items]
Stock split, ratio	3.33
Stock split, description	3 1/3-for-one
Number of operating segments | Segment	1
Cash equivalents	$ 315,500,000	$ 156,000,000
Accounts receivable	3,157,000	339,000
Impairment loss	$ 0	0	$ 0
Assumed dividend yield, percent	0.00%
Number of equity-based compensation awards | Award	8
IPO [Member]
Summary Of Significant Accounting Policies [Line Items]
Deferred public offering costs				$ 8,300,000
Vertex and Casebia [Member] | Collaborative Arrangement [Member]
Summary Of Significant Accounting Policies [Line Items]
Accounts receivable	$ 3,200,000
Allowance	$ 0
Vertex [Member] | Collaborative Arrangement [Member]
Summary Of Significant Accounting Policies [Line Items]
Accounts receivable		$ 300,000
Casebia Therapeutics LLP [Member]
Summary Of Significant Accounting Policies [Line Items]
Equity method investment, ownership percentage	50.00%				50.00%	50.00%