Unaudited Statements of Cash Flows (USD $)	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013
Cash flow from operating activities:
Net Income (Loss)	$ 17,128,755	$ (10,130,175)
Adjustments to reconcile net income (loss) to net cash provided by (used for) operating activities:
Cost of securities purchased	(620,940,648)	(445,926,657)
Proceeds from securities sold and matured	528,999,511	338,898,954
Net accretion of discount on United States Treasury Obligations	(62,787)	(67,130)
Net realized (gain) loss on United States Treasury Obligations	(476)	(3,745)
Net change in unrealized (gain) loss on United States Treasury Obligations and Futures	(7,892,580)	9,748,386
Change in operating receivables and liabilities:
Management fee payable	54,574	43,588
Brokerage fee payable	(2,480)	2,014
Net cash provided by (used for) operating activities	(82,716,131)	(107,434,765)
Cash flows from financing activities:
Proceeds from sale of Shares	117,920,685	165,681,440
Redemption of Shares	(34,726,792)	(66,825,306)
Net cash provided by (used for) financing activities	83,193,893	98,856,134
Net change in cash held by broker	477,762	(8,578,631)
Cash held by commodity broker at beginning of period	3,113,981	22,027,189
Cash held by commodity broker at end of period	$ 3,591,743	$ 13,448,558