Summary of Significant Accounting Policies - Additional Information (Details)		3 Months Ended
	Jan. 01, 2019 USD ($)	Mar. 31, 2019 USD ($) Segment	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments | Segment		1
Additional operating lease liabilities, net of deferred rent		$ 35,981,000
Operating lease, right-of-use asset		32,399,000
Prepaid expenses and other current assets		15,554,000		$ 15,724,000
Depreciation expense		400,000	$ 200,000
Otsuka U.S. Agreement
Summary Of Significant Accounting Policies [Line Items]
Cost share costs for license agreement		300,000	400,000
Reimbursable by Otsuka		200,000	100,000
Cost share costs by partner for license agreement		400,000	$ 0
Reimbursable to Otsuka		$ 200,000
Minimum
Summary Of Significant Accounting Policies [Line Items]
Property, plant and equipment, Useful life		3 years
Intangible assets, estimated useful life		4 years
Maximum
Summary Of Significant Accounting Policies [Line Items]
Property, plant and equipment, Useful life		7 years
Intangible assets, estimated useful life		9 years
ASC 842
Summary Of Significant Accounting Policies [Line Items]
Cumulative effect of adoption in retained earnings	$ 0
Additional operating lease liabilities, net of deferred rent	37,100,000
Operating lease, right-of-use asset	33,400,000
Prepaid expenses and other current assets	$ 900,000