Summary of Significant Accounting Policies - Additional Information (Detail)	1 Months Ended	3 Months Ended		12 Months Ended
	Jul. 31, 2016 USD ($)	Mar. 31, 2019 USD ($) Segment	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Jan. 01, 2019 USD ($)
Significant Accounting Policies [Line Items]
Cash and cash equivalents		$ 55,592,000		$ 56,992,000
Accumulated deficit		(456,597,000)		(450,565,000)
Working capital		$ 50,100,000
Number of operating segment | Segment		1
Cash and cash equivalents maturity period		Three months or less
Impairments of long-lived assets		$ 0		0
Revenue from product development award		$ 1,699,000
Type of Revenue [Extensible List]		us-gaap:GrantMember	us-gaap:GrantMember
Cash flows from financing activities			$ 1,551,000
Build-to-suit lease asset				8,651,000
Build-to-suit lease obligation				$ 7,324,000
Operating lease ROU assets		$ 9,799,000
Operating lease obligation		11,984,000
ASU 2016-02 [Member]
Significant Accounting Policies [Line Items]
Operating lease ROU assets					$ 10,400,000
Leasehold improvements					2,100,000
Operating lease obligation					12,600,000
ASU 2016-02 [Member] | Restatement Adjustment [Member]
Significant Accounting Policies [Line Items]
Accumulated deficit					1,300,000
Cash flows from investing activities			(1,500,000)
Cash flows from financing activities			$ 1,500,000
Deferred rent					(100,000)
ASU 2016-02 [Member] | Restatement Adjustment [Member] | 1020 Marsh Road, Menlo Park, California [Member]
Significant Accounting Policies [Line Items]
Build-to-suit lease asset					(8,600,000)
Build-to-suit lease obligation					$ (7,300,000)
Cancer Prevention & Research Institute of Texas [Member]
Significant Accounting Policies [Line Items]
Revenue from product development award	$ 20,000,000
Cancer Prevention & Research Institute of Texas [Member] | Aravive Biologics [Member]
Significant Accounting Policies [Line Items]
Revenue from product development award		$ 18,000,000
Type of Revenue [Extensible List]		us-gaap:GrantMember
Grant | Cancer Prevention & Research Institute of Texas [Member] | Aravive Biologics [Member]
Significant Accounting Policies [Line Items]
Deferred Revenue		$ 1,100,000
Assembled Workforce [Member]
Significant Accounting Policies [Line Items]
Estimated useful life of intangible assets		3 years
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