Summary of Significant Accounting Policies - Additional Information (Details)			1 Months Ended	9 Months Ended
	Jan. 01, 2018 USD ($)	Dec. 31, 2017 USD ($)	Feb. 28, 2017 USD ($)	Sep. 30, 2018 USD ($) Product	Sep. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Accumulated deficit		$ (150,333,000)		$ (176,671,000)
Deferred revenue				(22,254,000)	$ 38,299,000
Contract assets				0
Revenue recognized included in contract liability at beginning period				(31,000,000)
Revenue recognized from performance obligations satisfied in previous periods				$ 0
Federal statutory income tax rate		35.00%		21.00%
Estimated reduction in deferred tax assets due to remeasurement		$ 17,800,000
Incyte Collaboration Agreement
Summary Of Significant Accounting Policies [Line Items]
Collaboration agreement transaction price				$ 57,000,000
Upfront payment received			$ 45,000,000	45,000,000
Development milestone achieved				$ 12,000,000		$ 12,000,000
ASC 2014-09 | Incyte Collaboration Agreement
Summary Of Significant Accounting Policies [Line Items]
Collaboration agreement transaction price	$ 57,000,000
ASC 2014-09 | Incyte Corporation
Summary Of Significant Accounting Policies [Line Items]
Number of products covered under collaboration and licensing agreement | Product				1
Adjustments | ASC 2014-09
Summary Of Significant Accounting Policies [Line Items]
Accumulated deficit	(8,800,000)
Deferred revenue	$ (8,800,000)