Summary of Significant Accounting Policies - Additional Information (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) Segment DerivativeContract	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Significant Accounting Policies [Line Items]
Cash and cash equivalents	$ 62,605,000	$ 81,146,000	$ 118,783,000	$ 201,153,000
Accumulated deficit	(399,611,000)	(374,232,000)
Working capital	$ 59,900,000
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%
Aravive Biologics [Member]
Significant Accounting Policies [Line Items]
Cash and cash equivalents acquired	$ 5,700,000
Common Stock [Member]
Significant Accounting Policies [Line Items]
Reverse stock split ratio	1-for-6
Reverse stock split conversion ratio	0.16