Collaboration and Licensing Agreements - Additional Information (Details) - USD ($)		1 Months Ended		3 Months Ended		9 Months Ended
	Jan. 27, 2017	May 31, 2017	Feb. 28, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Jan. 01, 2018	Mar. 31, 2017
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Receivables from collaborations				$ 1,712,000		$ 1,712,000		$ 1,997,000
Mars, Inc.
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Potential license agreement fee						23,600,000
Additional potential payments based on sale of first licensed product						95,000,000
License fee				0		0
Incyte Collaboration Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of sharing in global development cost	30.00%
Future profits and losses share percentage	40.00%
Upfront payment received			$ 45,000,000
Development milestone achieved										$ 12,000,000
Development milestone received		$ 12,000,000
Additional potential development, regulatory and sales milestones receivable				418,000,000		418,000,000
Upfront consideration allocated to combined unit of accounting						45,000,000
Development milestone recognized						12,000,000
(Increase) decrease in research and development expenses				100,000	$ 600,000	600,000	$ 3,200,000
Receivables from collaborations				1,700,000		1,700,000		$ 2,000,000
Incyte Collaboration Agreement | ASC 606
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement transaction price									$ 57,000,000
Collaboration revenue recognized related to completion of combined performance obligation				$ 0	$ 0	$ 0	$ 22,300,000
Incyte Collaboration Agreement | Opt Out of Co-Funding Obligations
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional potential development, regulatory and commercialization milestones receivable	$ 738,000,000
Incremental royalty percentage on annual net sales	3.00%
Maximum incremental royalty, percentage of previous development expenditure	120.00%
Incyte Collaboration Agreement | Incyte
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of sharing in global development cost	70.00%
Future profits and losses share percentage	60.00%