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Investments in Affiliates -Summary of Transactions in, and Earnings from, Investments in Affiliates (Excluding Affiliated Money Market Fund (Detail) - USD ($)
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Investments in and Advances to Affiliates [Line Items]    
Beginning Value $ 102,216,801  
Change in Unrealized Appreciation (Depreciation) 7,523 $ (63,513)
Ending Value 99,259,143  
Affiliated Investments [Member]    
Investments in and Advances to Affiliates [Line Items]    
Beginning Value 102,216,801  
Ending Value 99,259,143  
Affiliated Investments [Member] | Exchange-Traded Fund [Member]    
Investments in and Advances to Affiliates [Line Items]    
Beginning Value [1] 79,818,080  
Change in Unrealized Appreciation (Depreciation) 37,850  
Ending Value [1] 79,855,930  
Dividend Income $ 461,186  
Affiliated Investments [Member] | PowerShares Treasury Collateral Portfolio [Member]    
Investments in and Advances to Affiliates [Line Items]    
Beginning Value   79,689,390
Change in Unrealized Appreciation (Depreciation)   310,370
Ending Value   $ 79,999,760
[1] Affiliated company. The security and the Fund are affiliated by having the same investment adviser. See Note 8.