Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018 USD ($) Segment	Jan. 01, 2019 USD ($)	Dec. 31, 2017 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Realized loss on marketable securities	$ 0
Marketable securities	$ 255,203,000		$ 0
Percentage of likelihood of realization required to record tax benefit	50.00%
Expenses incurred to obtain collaboration agreements and costs to fulfill contracts	$ 0
Costs to obtain or fulfill contract capitalized	$ 0
Number of reportable segment | Segment	1
Maximum [Member] | ASU 2016-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Maximum lease term	12 months
Maximum [Member] | ASU 2016-02 [Member] | Subsequent Event [Member]
Summary Of Significant Accounting Policies [Line Items]
Operating lease, liability		$ 21,000,000
Operating lease, right-of-use asset		23,000,000
Minimum [Member] | ASU 2016-02 [Member] | Subsequent Event [Member]
Summary Of Significant Accounting Policies [Line Items]
Operating lease, liability		19,000,000
Operating lease, right-of-use asset		$ 21,000,000