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Organization, Business, Basis of Presentation and Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Significant Accounting Policies [Line Items]          
Accumulated deficit $ 252,712   $ 252,712   $ 215,979
Cash, cash equivalents and available-for-sale investments $ 90,400   $ 90,400    
Weighted average shares subject to repurchase 175 21,871 4,607 29,467  
Pangu BioPharma [Member] | Hong Kong [Member]          
Significant Accounting Policies [Line Items]          
Majority-owned subsidiary percentage 98.00%   98.00%