Subsequent Events - Agreement with Gilead Sciences, Inc. (Detail) - USD ($)		3 Months Ended		6 Months Ended
	Jul. 19, 2019	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Subsequent Event [Line Items]
Total revenues		$ 3,985,000	$ 3,413,000	$ 8,116,000	$ 6,901,000
Agreement with Gilead [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Total revenues	$ 25,000,000
Agreement with Gilead [Member] | Maximum [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Performance milestone payments based on successful development	75,000,000
Agreement with Gilead [Member] | Sales-Based Milestones [Member] | Maximum [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Performance milestone payments based on successful development	70,000,000
Agreement with Gilead [Member] | Upfront Development Regulatory And Sales Based Milestone [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Performance milestone payments based on successful development	$ 150,000,000