Statements Of Financial Condition (USD $)	Sep. 30, 2012	Dec. 31, 2011
Assets
United States Treasury Obligations, at fair value (cost $1,629,756,268 and $1,959,877,867 respectively)	$ 1,629,797,437	$ 1,959,875,160
Cash held by broker	114,201,676	210,907,413
Net unrealized appreciation (depreciation) on futures contracts	175,469,983	(135,420,844)
Deposits with broker	1,919,469,096	2,035,361,729
Receivable for securities sold		11,999,926
Total assets	1,919,469,096	2,047,361,655
Liabilities
Payable for shares redeemed		11,525,820
Management fee payable	2,891,036	1,523,526
Brokerage fee payable	70	3,761
Total liabilities	2,891,106	13,053,107
Commitments and Contingencies (Note 9)
General shares:
Paid in capital-40 shares issued and outstanding as of September 30, 2012 and December 31, 2011, respectively	1,000	1,000
Accumulated earnings (deficit)	180	152
Total General shares	1,180	1,152
Shares:
Paid in capital-65,000,000 and 70,600,000 redeemable Shares issued and outstanding as of September 30, 2012 and December 31, 2011, respectively	2,129,667,932	2,280,227,892
Accumulated earnings (deficit)	(213,091,122)	(245,920,496)
Total Shares	1,916,576,810	2,034,307,396
Total shareholders' equity	1,916,577,990	2,034,308,548
Total liabilities and equity	$ 1,919,469,096	$ 2,047,361,655
Net asset value per share
General shares	$ 29.50	$ 28.80
Shares	$ 29.49	$ 28.81