Related Party Transactions - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended								12 Months Ended
	May 31, 2016	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Collaboration revenue		$ 10,936,000	$ 10,616,000	$ 11,118,000	$ 10,433,000	$ 7,880,000	$ 7,408,000	$ 7,677,000	$ 7,469,000	$ 43,103,000	$ 30,434,000	$ 26,117,000
Deferred revenue		28,810,000				55,932,000				28,810,000	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										18,500,000	10,300,000	$ 9,300,000
Accounts receivable		1,000,000.0				6,000,000.0				1,000,000.0	6,000,000.0
Deferred revenue		$ 0				$ 14,500,000				$ 0	$ 14,500,000
Novartis [Member] | Collaborative Arrangement [Member] | Minimum [Member]
Related Party Transaction [Line Items]
Percentage of shares owned		10.00%								10.00%