Commitment and Contingencies - Additional Information (Detail) - USD ($)	1 Months Ended			3 Months Ended		9 Months Ended
	Jul. 31, 2019	May 31, 2019	Apr. 30, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Feb. 28, 2017
Contingencies And Commitments [Line Items]
Research and development expense				$ 8,589,000	$ 6,158,000	$ 23,098,000	$ 15,657,000
Sponsored Research Agreement
Contingencies And Commitments [Line Items]
Purchase commitment, amount			$ 499,000
Purchase commitment, period			3 years
Research and development expense				41,000		124,000
Non-cancelable purchase commitments amount				98,000		98,000
Non-cancelable purchase commitments amount, 2019				42,000		42,000
Non-cancelable purchase commitments amount, 2020				56,000		56,000
Termination notice period		30 days
Maximum termination fees		$ 600,000
Indemnification Agreements
Contingencies And Commitments [Line Items]
Accrued liabilities				$ 0		$ 0		$ 0
Adimab Option and License Agreement
Contingencies And Commitments [Line Items]
Option fee including up front payment									$ 250,000
Potential milestone payments	$ 25,000,000