Summary of Significant Accounting Policies - Additional Information (Details)	1 Months Ended	12 Months Ended
	Jan. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) Segment	Dec. 31, 2017 USD ($) PerformanceObligation	Dec. 31, 2016 USD ($) PerformanceObligation
Summary Of Significant Accounting Policies [Line Items]
Number of reportable and operating segment | Segment		1
Property plant and equipment basis of valuation		cost
Property plant and equipment depreciation methods		the straight-line method over the estimated useful lives of the assets
Impairment of long-lived assets		$ 0
Deferred offering costs		0	$ 0
Proceeds from offering, net of underwriting discounts and commissions and offering expenses		$ 181,863,000	133,862,000	$ 61,148,000
Sanofi (Aventis Inc.) | Collaborative Agreement
Summary Of Significant Accounting Policies [Line Items]
Research and development expense reimbursing of preapproved costs		100.00%
Continuation payment, amount received	$ 45,000,000		$ 45,000,000	$ 35,000,000
Number of performance obligations | PerformanceObligation			3	3
Follow-on Public Offering
Summary Of Significant Accounting Policies [Line Items]
Deferred offering costs		$ 600,000	$ 400,000
Proceeds from offering, net of underwriting discounts and commissions and offering expenses		$ 182,500,000	$ 134,300,000
Minimum
Summary Of Significant Accounting Policies [Line Items]
Property plant and equipment useful life		2 years
Maximum
Summary Of Significant Accounting Policies [Line Items]
Property plant and equipment useful life		5 years
Maximum | ASU 2016-02
Summary Of Significant Accounting Policies [Line Items]
Short-term leases term		12 months
Leasehold improvements
Summary Of Significant Accounting Policies [Line Items]
Property plant and equipment estimated useful lives		shorter of their estimated useful lives or the related lease term