Statements Of Cash Flows (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Cash flows from operating activities:
Net Income (Loss)	$ 19,716,954	$ (8,156,864)	$ 96,510,718
Adjustments to reconcile net income (loss) to net cash provided by (used for) operating activities:
Cost of securities purchased	(1,359,748,435)	(1,959,759,293)	(1,066,655,997)
Proceeds from securities sold and matured	1,519,986,521	1,855,993,162	990,994,789
Net accretion of discount on United States Treasury Obligations	(222,165)	(264,484)	(305,721)
Net realized (gain) loss on United States Treasury Obligations	(2,188)	(14,513)	875
Net change in unrealized (gain) loss on United States Treasury Obligations and futures	(39,748,121)	72,619,029	(8,433,348)
Change in operating receivables and liabilities:
Receivable for securities sold	10,999,933	(10,999,933)
Payable to broker			(9,709,456)
Management fee payable	(94,880)	84,879	77,990
Brokerage fee payable	(6,696)	2,360	2,554
Net cash provided by (used for) operating activities	150,880,923	(50,495,657)	2,482,404
Cash flows from financing activities:
Proceeds from sale of Shares	83,486,258	427,325,248	151,946,388
Payments for redemption of Shares	(214,142,424)	(358,403,156)	(107,687,800)
Redemption of non-controlling interest			(2,073)
Net cash provided by (used for) financing activities	(130,656,166)	68,922,092	44,256,515
Net change in cash held by broker	20,224,757	18,426,435	46,738,919
Cash held by broker at beginning of period	65,165,354	46,738,919
Cash held by broker at end of period	$ 85,390,111	$ 65,165,354	$ 46,738,919