Unaudited Statement of Changes in Shareholders' Equity (USD $)	Total USD ($)	General Shares [Member]	General Shares [Member] Total Equity [Member] USD ($)	Shares [Member]	Shares [Member] Total Equity [Member] USD ($)
Balance - value at Mar. 31, 2013	$ 227,252,996		$ 1,165		$ 227,251,831
Balance - shares at Mar. 31, 2013		40		7,800,000
Sale of Shares, shares				2,800,000
Sale of Shares, Value	79,259,944				79,259,944
Redemption of Shares, shares				(2,200,000)
Redemption of Shares, Value	(61,074,544)				(61,074,544)
Net Increase (Decrease) due to Share Transactions, value	18,185,400				18,185,400
Net Increase (Decrease) due to Share Transactions, shares				600,000
Net Income (Loss)
Net investment income (loss)	(397,282)		(1)		(397,281)
Net realized gain (loss) on United States Treasury Obligations and Futures	(206,366)		(1)		(206,365)
Net change in unrealized gain (loss) on United States Treasury Obligations and Futures	(16,182,246)		(74)		(16,182,172)
Net Income (Loss)	(16,785,894)		(76)		(16,785,818)
Net Change in Shareholders' Equity, value	1,399,506		(76)		1,399,582
Net Change in Shareholders' Equity, shares				600,000
Balance - value at Jun. 30, 2013	228,652,502		1,089		228,651,413
Balance - shares at Jun. 30, 2013		40		8,400,000
Balance - value at Dec. 31, 2012	139,926,543		1,120		139,925,423
Balance - shares at Dec. 31, 2012		40		5,000,000
Sale of Shares, shares				5,800,000
Sale of Shares, Value	165,681,440				165,681,440
Redemption of Shares, shares				(2,400,000)
Redemption of Shares, Value	(66,825,306)				(66,825,306)
Net Increase (Decrease) due to Share Transactions, value	98,856,134				98,856,134
Net Increase (Decrease) due to Share Transactions, shares				3,400,000
Net Income (Loss)
Net investment income (loss)	(697,415)		(4)		(697,411)
Net realized gain (loss) on United States Treasury Obligations and Futures	315,626		3		315,623
Net change in unrealized gain (loss) on United States Treasury Obligations and Futures	(9,748,386)		(30)		(9,748,356)
Net Income (Loss)	(10,130,175)		(31)		(10,130,144)
Net Change in Shareholders' Equity, value	88,725,959		(31)		88,725,990
Net Change in Shareholders' Equity, shares				3,400,000
Balance - value at Jun. 30, 2013	228,652,502		1,089		228,651,413
Balance - shares at Jun. 30, 2013		40		8,400,000
Balance - value at Dec. 31, 2013	268,283,601		1,166		268,282,435
Balance - shares at Dec. 31, 2013		40		9,200,000
Sale of Shares, shares				4,000,000
Sale of Shares, Value	117,920,685				117,920,685
Redemption of Shares, shares	(200,000)			(1,200,000)
Redemption of Shares, Value	(34,726,792)				(34,726,792)
Net Increase (Decrease) due to Share Transactions, value	83,193,893				83,193,893
Net Increase (Decrease) due to Share Transactions, shares				2,800,000
Net Income (Loss)
Net investment income (loss)	(1,058,934)		(4)		(1,058,930)
Net realized gain (loss) on United States Treasury Obligations and Futures	10,295,109		38		10,295,071
Net change in unrealized gain (loss) on United States Treasury Obligations and Futures	7,892,580		29		7,892,551
Net Income (Loss)	17,128,755		63		17,128,692
Net Change in Shareholders' Equity, value	100,322,648		63		100,322,585
Net Change in Shareholders' Equity, shares				2,800,000
Balance - value at Jun. 30, 2014	368,606,249		1,229		368,605,020
Balance - shares at Jun. 30, 2014		40		12,000,000
Balance - value at Mar. 31, 2014	292,750,780		1,171		292,749,609
Balance - shares at Mar. 31, 2014		40		10,000,000
Sale of Shares, shares				2,400,000
Sale of Shares, Value	71,042,717				71,042,717
Redemption of Shares, shares				(400,000)
Redemption of Shares, Value	(11,956,586)				(11,956,586)
Net Increase (Decrease) due to Share Transactions, value	59,086,131				59,086,131
Net Increase (Decrease) due to Share Transactions, shares				2,000,000
Net Income (Loss)
Net investment income (loss)	(602,646)		(2)		(602,644)
Net realized gain (loss) on United States Treasury Obligations and Futures	5,906,206		20		5,906,186
Net change in unrealized gain (loss) on United States Treasury Obligations and Futures	11,465,778		40		11,465,738
Net Income (Loss)	16,769,338		58		16,769,280
Net Change in Shareholders' Equity, value	75,855,469		58		75,855,411
Net Change in Shareholders' Equity, shares				2,000,000
Balance - value at Jun. 30, 2014	$ 368,606,249		$ 1,229		$ 368,605,020
Balance - shares at Jun. 30, 2014		40		12,000,000