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Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)
9 Months Ended
Sep. 30, 2017
Dec. 31, 2016
Summary Of Significant Accounting Policies [Line Items]    
Transfers between Levels 1, 2 and 3 $ 0 $ 0
Sale of available for sale securities 0  
Aggregate fair value of available for sale securities held for less than twelve months 1,800,000  
Significant Unobservable Inputs (Level 3) [Member]    
Summary Of Significant Accounting Policies [Line Items]    
Assets, fair value disclosure, recurring 0 0
Liabilities, fair value disclosure, recurring $ 0 $ 0