Summary of Significant Accounting Policies - Additional Information (Detail)	1 Months Ended	6 Months Ended	12 Months Ended
	Nov. 04, 2016 USD ($)	Jun. 30, 2017 USD ($) Segment	Dec. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)
Significant Accounting Policies [Line Items]
Cash and cash equivalents		$ 143,358,000	$ 201,153,000	$ 137,497,000	$ 182,069,000
Accumulated deficit		(355,572,000)	(289,253,000)
Working capital		$ 90,300,000
Proceeds from issuance of common stock in follow-on offering, net of issuance costs	$ 59,100,000
Number of operating segment | Segment		1
Cash and cash equivalents maturity period		Three months or less
Impairments of long-lived assets		$ 0	$ 0
Minimum [Member]
Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives		3 years
Maximum [Member]
Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives		5 years
Percentage for the amount of benefit realized upon ultimate settlement		50.00%