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Investments in Affiliates - Summary of Transactions in, and Earnings from, Investments in Affiliates (Excluding Affiliated Money Market Funds) (Detail) - USD ($)
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Investments In And Advances To Affiliates [Line Items]    
Beginning Value $ 395,084,923  
Change in Unrealized Appreciation (Depreciation) 117,402 $ (343,649)
Ending Value 319,596,529  
Affiliated Investments [Member]    
Investments In And Advances To Affiliates [Line Items]    
Beginning Value 395,084,923  
Ending Value 319,596,529  
Affiliated Investments [Member] | Invesco Treasury Collateral ETF [Member]    
Investments In And Advances To Affiliates [Line Items]    
Beginning Value [1] 250,145,856  
Change in Unrealized Appreciation (Depreciation) 118,620  
Ending Value [1] 250,264,476  
Dividend Income $ 1,445,333  
Affiliated Investments [Member] | PowerShares Treasury Collateral Portfolio [Member]    
Investments In And Advances To Affiliates [Line Items]    
Beginning Value   249,742,548
Change in Unrealized Appreciation (Depreciation)   972,684
Ending Value   $ 250,715,232
[1] Affiliated company. The security and the Fund are affiliated by having the same investment adviser. See Note 8.