Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) Segment DerivativeContract	Dec. 31, 2016 USD ($) DerivativeContract	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Significant Accounting Policies [Line Items]
Accumulated deficit	$ (374,232,000)	$ (289,253,000)
Working capital	76,100,000
Cash and cash equivalents	$ 81,146,000	201,153,000	$ 182,069,000	$ 170,566,000
Proceeds from issuance of common stock in follow-on offering, net of issuance costs		59,136,000	80,209,000
Number of operating segment | Segment	1
Cash and cash equivalents maturity period	Three months or less
Impairments of long-lived assets	$ 0	$ 0	$ 0
Teijin Limited [Member]
Significant Accounting Policies [Line Items]
Contract revenues received	$ 40,000,000
Level II Assets [Member]
Significant Accounting Policies [Line Items]
Number of foreign currency derivative contracts outstanding | DerivativeContract	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%