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Related Party Transactions - Additional Information (Detail) - Smith, Gambrell & Russell, LLP [Member] - USD ($)
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Related Party Transaction [Line Items]      
Fees payable to Smith, Gambrell & Russell $ 400,000 $ 500,000 $ 300,000
Amount due to Smith, Gambrell & Russell $ 0 $ 0