Summary of Significant Accounting Policies - Additional Information (Details) - USD ($)	1 Months Ended		3 Months Ended	6 Months Ended
	Jan. 31, 2018	Oct. 31, 2017	Jun. 30, 2019	Jun. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
Summary Of Significant Accounting Policies [Line Items]
Operating lease right-of-use assets			$ 29,058,000	$ 29,058,000
Lease liabilities			47,930,000	47,930,000
Deferred Rent						$ 7,478,000
AbbVie
Summary Of Significant Accounting Policies [Line Items]
Upfront payments		$ 205,000,000
Upfront payment received	$ 200,000,000	5,000,000
Exclusive option rights exercised for each program		250,000,000
Collaboration revenue			6,900,000	12,500,000
Deferred revenue			$ 162,100,000	162,100,000
AbbVie | Maximum
Summary Of Significant Accounting Policies [Line Items]
Additional milestone payments per program related to initiation of certain clinical studies and regulatory approval		$ 242,800,000
Accounting Standards Update 2016-02
Summary Of Significant Accounting Policies [Line Items]
Operating lease right-of-use assets					$ 31,400,000
Lease liabilities					$ 38,900,000
Deferred Rent						$ 7,500,000
Money Market Funds
Summary Of Significant Accounting Policies [Line Items]
Unrealized gains or loss on money market funds				$ 0