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Significant Accounting Policies - Additional Information (Detail)
shares in Millions
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2019
shares
Sep. 30, 2018
shares
Sep. 30, 2019
Segment
shares
Sep. 30, 2018
shares
Dec. 31, 2017
Accounting Policies [Abstract]          
Number of operating segment | Segment     1    
Short term investments maturity     12 months    
Anti-dilutive shares | shares 9.2 9.3 3.6 9.1  
U.S. federal statutory income tax rate     21.00%   35.00%