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Summary of Significant Accounting Policies - Additional Information (Detail)	1 Months Ended	6 Months Ended	12 Months Ended
	Nov. 04, 2016 USD ($)	Jun. 30, 2018 USD ($) Segment DerivativeContract	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Significant Accounting Policies [Line Items]
Cash and cash equivalents		$ 67,776,000	$ 81,146,000	$ 143,358,000	$ 201,153,000
Accumulated deficit		(393,061,000)	(374,232,000)
Working capital		$ 64,400,000
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
Teijin Limited [Member]
Significant Accounting Policies [Line Items]
Contract revenues received		40,000,000
Reclassification of Restricted Cash to Cash and Cash Equivalents [Member] \| ASU-2016-18 [Member]
Significant Accounting Policies [Line Items]
Reclassification of restricted cash		$ 2,400,000
Level II Assets [Member]
Significant Accounting Policies [Line Items]
Number of foreign currency derivative contracts outstanding \| DerivativeContract		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%