Grant Revenues - Additional Information (Details) - USD ($)		3 Months Ended		9 Months Ended		48 Months Ended
	Jun. 01, 2014	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	May 31, 2018	Dec. 31, 2017
Research And Development Arrangement Contract To Perform For Others [Line Items]
Grant revenues		$ 0	$ 1,261,000	$ 3,888,000	$ 3,723,000
Type of revenue [extensible list]		us-gaap:GrantMember	us-gaap:GrantMember	us-gaap:GrantMember	us-gaap:GrantMember
Outstanding grant receivable		$ 2,422,000		$ 2,422,000			$ 3,078,000
Deferred revenue		0		0			$ 20,000
Remaining grant receivable		$ 2,400,000		$ 2,400,000
CPRIT
Research And Development Arrangement Contract To Perform For Others [Line Items]
Grant contract term	4 years
Grant contract beginning date	Jun. 01, 2014
Grant contract expiration date	May 31, 2018
Grant revenues						$ 19,800,000
Type of revenue [extensible list]						us-gaap:GrantMember
CPRIT | Maximum
Research And Development Arrangement Contract To Perform For Others [Line Items]
Research grant contract amount	$ 19,800,000