Summary of Significant Accounting Policies - Additional Information (Details)		3 Months Ended					12 Months Ended
	Jan. 02, 2017 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) Segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Net operating loss deferred tax asset		$ 42,335,000				$ 84,372,000	$ 42,335,000	$ 84,372,000
Collaboration revenue		87,316,000	$ 41,283,000	$ 28,520,000	$ 20,865,000	1,535,000	$ 177,984,000	1,535,000
Number of operating segments | Segment							1
Write-offs of bad debt							$ 0	0
Allowance for doubtful accounts		0				0	0	0
Significant uncertain tax positions		0				$ 0	$ 0	0
Expected dividend yield							0.00%
Depreciation expense							$ 616,000	295,000	$ 96,000
Minimum
Summary Of Significant Accounting Policies [Line Items]
Property, plant and equipment, Useful life							3 years
Maximum
Summary Of Significant Accounting Policies [Line Items]
Property, plant and equipment, Useful life							7 years
Otsuka U.S. Agreement
Summary Of Significant Accounting Policies [Line Items]
Cost share costs for license agreement							$ 700,000
Reimbursable by Otsuka							200,000
ASU 2016-09
Summary Of Significant Accounting Policies [Line Items]
Net operating loss deferred tax asset		400,000					400,000
Offsetting valuation allowance		$ 400,000					400,000
Cumulative-effect adjustment to reduce retained earnings	$ 200,000
ASC 606
Summary Of Significant Accounting Policies [Line Items]
Increase in current assets							3,200,000	0
Changes in current liabilities							0	0
ASC 606 | Collaboration Agreements | Otsuka Pharmaceutical Company. Ltd.
Summary Of Significant Accounting Policies [Line Items]
Collaboration revenue							0	0
ASC 606 | Collaboration Agreements | Mitsubishi Tanabe Pharma Corporation
Summary Of Significant Accounting Policies [Line Items]
Increase in revenue							$ 3,200,000	$ 0