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Fair Value Of Financial Instruments (Valuation of Securities Using Fair Value Hierarchy) (Detail) - USD ($)
Jun. 30, 2015
Dec. 31, 2014
Short-Term Investments [Member]    
Schedule of Trading Securities and Other Trading Assets [Line Items]    
Securities, fair value $ 1,831,601,309 $ 581,778,927
Exchange-Traded Futures Contracts [Member] | United States [Member]    
Schedule of Trading Securities and Other Trading Assets [Line Items]    
Securities, fair value (47,173,255) (167,617,930)
Fair Value, Inputs, Level 1 [Member] | Short-Term Investments [Member]    
Schedule of Trading Securities and Other Trading Assets [Line Items]    
Securities, fair value 1,831,601,309 581,778,927
Fair Value, Inputs, Level 1 [Member] | Exchange-Traded Futures Contracts [Member] | United States [Member]    
Schedule of Trading Securities and Other Trading Assets [Line Items]    
Securities, fair value (47,173,255) (167,617,930)
Fair Value, Inputs, Level 2 [Member] | Short-Term Investments [Member]    
Schedule of Trading Securities and Other Trading Assets [Line Items]    
Securities, fair value 0 0
Fair Value, Inputs, Level 2 [Member] | Exchange-Traded Futures Contracts [Member] | United States [Member]    
Schedule of Trading Securities and Other Trading Assets [Line Items]    
Securities, fair value 0 0
Fair Value, Inputs, Level 3 [Member] | Short-Term Investments [Member]    
Schedule of Trading Securities and Other Trading Assets [Line Items]    
Securities, fair value 0 0
Fair Value, Inputs, Level 3 [Member] | Exchange-Traded Futures Contracts [Member] | United States [Member]    
Schedule of Trading Securities and Other Trading Assets [Line Items]    
Securities, fair value $ 0 $ 0