Organization and Description of Business (Detail) $ in Thousands	Dec. 31, 2020 USD ($) divestiture program	Dec. 31, 2019 USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of development programs underway with other research institutions \| program	2
Total cash, cash equivalents and marketable securities at fair value	$ 505,109	$ 251,155
Number of discrete business divestitures \| divestiture	2
Accumulated deficit	$ 1,324,840	$ 1,115,457

The cumulative amount of the reporting entity's undistributed earnings or deficit.

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