Related Party Transactions - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended		9 Months Ended
	May 31, 2016	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jun. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Collaboration revenue		$ 10,616,000	$ 7,408,000	$ 32,167,000	$ 22,554,000
Deferred revenue		35,144,000	46,264,000	35,144,000	46,264,000		$ 55,932,000	$ 59,868,000
Novartis [Member] | Collaborative Arrangement [Member]
Related Party Transaction [Line Items]
Issuance of common stock through at-the-market offering, net of issuance costs, shares	277,777
Collaboration revenue		4,800,000	$ 2,400,000	14,300,000	$ 7,200,000
Accounts receivable		1,000,000.0		1,000,000.0			6,000,000.0
Deferred revenue		$ 3,100,000		$ 3,100,000			$ 14,500,000
Novartis [Member] | Collaborative Arrangement [Member] | Minimum [Member]
Related Party Transaction [Line Items]
Percentage of shares owned						10.00%