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Investments in Affiliates - Summary of Transactions in, and Earnings from, Investments in PowerShares Treasury Collateral Portfolio (Detail) - USD ($)
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Investments in and Advances to Affiliates [Line Items]    
Beginning Value $ 131,029,805  
Change in Unrealized Appreciation (Depreciation) (116,229) $ 178,110
Ending Value 346,000,956  
Affiliated Investments [Member]    
Investments in and Advances to Affiliates [Line Items]    
Ending Value 346,000,956  
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) 84,458  
Ending Value [1] $ 250,074,684  
[1] Affiliated company. The security and the Fund are affiliated by having the same investment adviser. See Note 6.