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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017	Mar. 31, 2017
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Receivables from collaborations				$ 1,809,000		$ 1,809,000			$ 1,142,000
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
Initial 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			45,000,000
Development milestone achieved				12,000,000		12,000,000				$ 12,000,000
Development milestone received		$ 12,000,000
Additional potential development, regulatory and sales milestones receivable				418,000,000		$ 418,000,000
Future net profits and losses share percentage						40.00%
Profit and loss sharing description						Incyte and the Company will share in any future United States net profits and losses, with the Company bearing 40% and Incyte bearing 60%, respectively, and outside the United States the Company will be eligible to receive from Incyte tiered royalties, with rates in the low to mid-teens of sales.
Upfront consideration allocated to combined unit of accounting						$ 45,000,000
Development milestone recognized						12,000,000
Amortization of upfront fee and milestone					$ 7,300,000		$ 18,700,000
Collaboration agreement transaction price				57,000,000		57,000,000
Reduction of research and development expenses				600,000	$ 2,000,000	3,200,000	$ 5,700,000
Receivables from collaborations				1,700,000		1,700,000
Incyte Collaboration Agreement | ASC 2014-09
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		$ 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%
Future net profits and losses share percentage						60.00%