Statement Of Changes In Shareholders' Equity (USD $)	Total USD ($)	General Shares [Member]	General Shares [Member] Paid In Capital [Member] USD ($)	General Shares [Member] Accumulated Earnings (Deficit) [Member] USD ($)	General Shares [Member] Total Equity [Member] USD ($)	Shares [Member]	Shares [Member] Paid In Capital [Member] USD ($)	Shares [Member] Accumulated Earnings (Deficit) [Member] USD ($)	Shares [Member] Total Equity [Member] USD ($)
Balance, shares at Jun. 30, 2011		40				96,000,000
Balance, value at Jun. 30, 2011	$ 3,072,232,373		$ 1,000	$ 280	$ 1,280		$ 3,035,280,044	$ 36,951,049	$ 3,072,231,093
Sale of Shares, shares						6,200,000
Sale of Shares, value	207,639,784						207,639,784		207,639,784
Redemption of Shares, shares						(16,800,000)
Redemption of Shares, value	(529,143,018)						(529,143,018)		(529,143,018)
Net Income (Loss)
Net investment income (loss)	(6,527,788)			(23)	(23)			(6,527,765)	(6,527,765)
Net realized gain (loss) on United States Treasury Obligations and Futures	(68,807,152)			(9)	(9)			(68,807,143)	(68,807,143)
Net change in unrealized gain (loss) on United States Treasury Obligations and Futures	(118,908,092)			(50)	(50)			(118,908,042)	(118,908,042)
Net Income (Loss)	(194,243,032)			(82)	(82)			(194,242,950)	(194,242,950)
Balance, shares at Sep. 30, 2011		40				85,400,000
Balance, value at Sep. 30, 2011	2,556,486,107		1,000	198	1,198		2,713,776,810	(157,291,901)	2,556,484,909
Balance, shares at Dec. 31, 2010		40				83,800,000
Balance, value at Dec. 31, 2010	2,712,709,208		1,000	295	1,295		2,601,330,830	111,377,083	2,712,707,913
Sale of Shares, shares						55,600,000
Sale of Shares, value	1,889,658,574						1,889,658,574		1,889,658,574
Redemption of Shares, shares						(54,000,000)
Redemption of Shares, value	(1,777,212,594)						(1,777,212,594)		(1,777,212,594)
Net Income (Loss)
Net investment income (loss)	(20,769,202)			(27)	(27)			(20,769,175)	(20,769,175)
Net realized gain (loss) on United States Treasury Obligations and Futures	222,646,914			121	121			222,646,793	222,646,793
Net change in unrealized gain (loss) on United States Treasury Obligations and Futures	(470,546,793)			(191)	(191)			(470,546,602)	(470,546,602)
Net Income (Loss)	(268,669,081)			(97)	(97)			(268,668,984)	(268,668,984)
Balance, shares at Sep. 30, 2011		40				85,400,000
Balance, value at Sep. 30, 2011	2,556,486,107		1,000	198	1,198		2,713,776,810	(157,291,901)	2,556,484,909
Balance, shares at Dec. 31, 2011		40				70,600,000
Balance, value at Dec. 31, 2011	2,034,308,548		1,000	152	1,152		2,280,227,892	(245,920,496)	2,034,307,396
Sale of Shares, shares						9,600,000
Sale of Shares, value	278,127,646						278,127,646		278,127,646
Redemption of Shares, shares						(15,200,000)
Redemption of Shares, value	(428,687,606)						(428,687,606)		(428,687,606)
Net Income (Loss)
Net investment income (loss)	(12,702,516)			(11)	(11)			(12,702,505)	(12,702,505)
Net realized gain (loss) on United States Treasury Obligations and Futures	(265,402,785)			(130)	(130)			(265,402,655)	(265,402,655)
Net change in unrealized gain (loss) on United States Treasury Obligations and Futures	310,934,703			169	169			310,934,534	310,934,534
Net Income (Loss)	32,829,402			28	28			32,829,374	32,829,374
Balance, shares at Sep. 30, 2012		40				65,000,000
Balance, value at Sep. 30, 2012	1,916,577,990		1,000	180	1,180		2,129,667,932	(213,091,122)	1,916,576,810
Balance, shares at Jun. 30, 2012		40				65,200,000
Balance, value at Jun. 30, 2012	1,835,038,232		1,000	126	1,126		2,136,001,891	(300,964,785)	1,835,037,106
Sale of Shares, shares						4,200,000
Sale of Shares, value	124,516,164						124,516,164		124,516,164
Redemption of Shares, shares						(4,400,000)
Redemption of Shares, value	(130,850,123)						(130,850,123)		(130,850,123)
Net Income (Loss)
Net investment income (loss)	(4,208,559)			(7)	(7)			(4,208,552)	(4,208,552)
Net realized gain (loss) on United States Treasury Obligations and Futures	34,867,751			12	12			34,867,739	34,867,739
Net change in unrealized gain (loss) on United States Treasury Obligations and Futures	57,214,525			49	49			57,214,476	57,214,476
Net Income (Loss)	87,873,717			54	54			87,873,663	87,873,663
Balance, shares at Sep. 30, 2012		40				65,000,000
Balance, value at Sep. 30, 2012	$ 1,916,577,990		$ 1,000	$ 180	$ 1,180		$ 2,129,667,932	$ (213,091,122)	$ 1,916,576,810