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Investments in Affiliates - Summary of Transactions in, and Earnings from, Investments in Affiliates (Excluding Affiliated Money Market Funds) (Detail) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Investments In And Advances To Affiliates [Line Items]      
Beginning Value $ 131,029,805    
Change in Unrealized Appreciation (Depreciation) (40,787) $ (56,173) $ (53,402)
Ending Value 317,746,227 $ 131,029,805  
Affiliated Investments [Member]      
Investments In And Advances To Affiliates [Line Items]      
Ending Value 317,746,227    
Affiliated Investments [Member] | Exchange-Traded Fund - PowerShares Treasury Collateral Portfolio [Member]      
Investments In And Advances To Affiliates [Line Items]      
Purchases at Cost 249,990,226    
Change in Unrealized Appreciation (Depreciation) (247,678)    
Ending Value [1] 249,742,548    
Dividend Income $ 1,771,803    
[1] Affiliated company. The security and the Fund are affiliated by having the same investment adviser. See Note 8.