Summary of Significant Accounting Policies and Basis of Presentation - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2019 USD ($) Segment	Dec. 12, 2019	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Mar. 31, 2016	Feb. 12, 2016
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments | Segment	1
Cash and cash equivalents	$ 943,771,000			$ 456,649,000	$ 239,758,000
Restricted Cash	5,041,000			3,163,000	3,154,000
Significant uncertain tax positions	0			0	$ 0
Net lease assets	41,502,000
Net lease liabilities	52,539,000
ASC 842 [Member]
Summary Of Significant Accounting Policies [Line Items]
Net lease assets			$ 26,087,000
Net lease liabilities			$ 37,600,000
Vertex and Casebia [Member] | Collaborative Arrangement [Member]
Summary Of Significant Accounting Policies [Line Items]
Other receivables	$ 4,100,000			$ 3,400,000
Casebia Therapeutics Limited Liability Partnership [Member]
Summary Of Significant Accounting Policies [Line Items]
Equity method investment, ownership percentage		50.00%				50.00%	50.00%