Statement Of Changes In Shareholders' Equity (USD $)	Total	Total Shareholders' Equity [Member]	Non-controlling Interest	General Shares [Member]	Shares [Member]
Beginning Balance, shares at Dec. 31, 2009				40	7,000,000
Beginning Balance, value at Dec. 31, 2009	$ 263,428,759	$ 263,427,254	$ 1,505	$ 1,505	$ 263,425,749
Sale of Shares, value	151,946,388	151,946,388			151,946,388
Sale of Shares, shares					3,600,000
Redemption of Shares, value	(107,687,800)	(107,687,800)			(107,687,800)
Redemption of Shares, shares					(2,800,000)
Net Income (Loss)
Net investment income (loss)	(1,979,135)	(1,979,123)	(12)	(12)	(1,979,111)
Net realized gain (loss) on United States Treasury Obligations and Futures	90,056,505	90,055,975	530	530	90,055,445
Net change in unrealized gain (loss) on United States Treasury Obligations and Futures	8,433,348	8,433,298	50	50	8,433,248
Net Income (Loss)	96,510,718	96,510,150	568	568	96,509,582
Redemption of non-controlling interest	(2,073)		(2,073)
Ending Balance, shares at Dec. 31, 2010				40	7,800,000
Ending Balance, value at Dec. 31, 2010	404,195,992	404,195,992		2,073	404,193,919
Sale of Shares, value		427,325,248			427,325,248
Sale of Shares, shares					7,200,000
Redemption of Shares, value		(369,216,158)			(369,216,158)
Redemption of Shares, shares					(6,600,000)
Net Income (Loss)
Net investment income (loss)	(3,571,953)	(3,571,953)		(3)	(3,571,950)
Net realized gain (loss) on United States Treasury Obligations and Futures		68,034,118		476	68,033,642
Net change in unrealized gain (loss) on United States Treasury Obligations and Futures		(72,619,029)		(383)	(72,618,646)
Net Income (Loss)	(8,156,864)	(8,156,864)		90	(8,156,954)
Ending Balance, shares at Dec. 31, 2011	8,400,000			40	8,400,000
Ending Balance, value at Dec. 31, 2011		454,148,218		2,163	454,146,055
Sale of Shares, value		83,486,258			83,486,258
Sale of Shares, shares					1,400,000
Redemption of Shares, value		(203,329,422)			(203,329,422)
Redemption of Shares, shares					(3,600,000)
Net Income (Loss)
Net investment income (loss)	(2,671,730)	(2,671,730)		(16)	(2,671,714)
Net realized gain (loss) on United States Treasury Obligations and Futures		(17,359,437)		(107)	(17,359,330)
Net change in unrealized gain (loss) on United States Treasury Obligations and Futures		39,748,121		244	39,747,877
Net Income (Loss)	19,716,954	19,716,954		121	19,716,833
Ending Balance, shares at Dec. 31, 2012	6,200,000			40	6,200,000
Ending Balance, value at Dec. 31, 2012		$ 354,022,008		$ 2,284	$ 354,019,724