Summary of Significant Accounting Policies - Fair Value of Derivative Instruments and Effect of Derivative Instruments on Statements of Income and Expenses (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Net Unrealized Appreciation (Depreciation) on Futures Contracts	$ 21,250,208		$ 21,250,208		$ 13,354,799
Net Realized Gain (Loss) on Futures	5,906,143	(210,017)	10,294,633	311,881
Net Change in Unrealized Gain (Loss) on Futures	$ 11,470,118	$ (16,179,726)	$ 7,895,409	$ (9,747,415)