Schedule of Investments (Unaudited) - Treasury Bills - USD ($)		Jun. 30, 2016		Dec. 31, 2015
Schedule of Investments [Line Items]
Fair Value		$ 789,905,910		$ 1,077,913,905
U.S. Treasury Bills, 0.19% due July 07, 2016 [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity		45.91%
Fair Value		$ 383,996,928
Face Value		$ 384,000,000
U.S. Treasury Bills, 0.26% due July 28, 2016 [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity		15.54%
Fair Value		$ 129,983,230
Face Value		$ 130,000,000
U.S. Treasury Bills, 0.25% due July 21, 2016 [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity		13.15%
Fair Value		$ 109,989,330
Face Value		$ 110,000,000
U.S. Treasury Bills, 0.34% due September 01, 2016 [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity	[1]	19.84%
Fair Value	[1]	$ 165,936,422
Face Value	[1]	$ 166,000,000
United States Treasury Obligations [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity		99.44%	[2]	95.09%	[3]
Fair Value		$ 789,905,910	[2]	$ 1,077,913,905	[3]
U.S. Treasury Bills, 0.235% due January 7, 2016 [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity				12.35%
Fair Value				$ 139,999,440
Face Value				$ 140,000,000
U.S. Treasury Bills, 0.205% due January 14, 2016 [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity				17.64%
Fair Value				$ 199,994,400
Face Value				$ 200,000,000
U.S. Treasury Bills, 0.195% due January 21, 2016 [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity	[4]			13.67%
Fair Value	[4]			$ 154,993,645
Face Value	[4]			$ 155,000,000
U.S. Treasury Bills, 0.170% due January 28, 2016 [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity				17.64%
Fair Value				$ 199,983,600
Face Value				$ 200,000,000
U.S. Treasury Bills, 0.200% due February 4, 2016 [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity				2.03%
Fair Value				$ 22,998,206
Face Value				$ 23,000,000
U.S. Treasury Bills, 0.135% due February 11, 2016 [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity				8.82%
Fair Value				$ 99,986,800
Face Value				$ 100,000,000
U.S. Treasury Bills, 0.145% due February 18, 2016 [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity				9.35%
Fair Value				$ 105,986,008
Face Value				$ 106,000,000
U.S. Treasury Bills, 0.140% due February 25, 2016 [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity				0.27%
Fair Value				$ 2,999,706
Face Value				$ 3,000,000
U.S. Treasury Bills, 0.215% due March 3, 2016 [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity				5.73%
Fair Value				$ 64,989,730
Face Value				$ 65,000,000
U.S. Treasury Bills, 0.280% due March 10, 2016 [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity				7.59%
Fair Value				$ 85,982,370
Face Value				$ 86,000,000

[1] United States Treasury Obligations of $22,991,107 are on deposit with the Commodity Broker and held as maintenance margin for open futures contracts.

[2] Security may be traded on a discount basis. The interest rate shown represents the discount rate at the most recent auction date of the security prior to period end.

[3] Security may be traded on a discount basis. The interest rate shown represents the discount rate at the most recent auction date of the security prior to year end.

[4] United States Treasury Obligations of $34,998,099 are on deposit with the Commodity Broker and held as maintenance margin for open futures contracts.