Schedule of Investments (Unaudited) - Treasury Bills - USD ($)		Mar. 31, 2016		Dec. 31, 2015
Schedule of Investments [Line Items]
Fair Value	[1]	$ 768,801,792		$ 1,077,913,905
U.S. Treasury Bills, 0.275% due April 7, 2016 [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity		20.27%
Fair Value		$ 165,997,012
Face Value		$ 166,000,000
U.S. Treasury Bills, 0.270% due April 21, 2016 [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity		9.28%
Fair Value		$ 75,995,820
Face Value		$ 76,000,000
U.S. Treasury Bills, 0.200% due April 28, 2016 [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity		17.95%
Fair Value		$ 146,981,772
Face Value		$ 147,000,000
U.S. Treasury Bills, 0.325% due June 2, 2016 [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity	[2]	21.00%
Fair Value	[2]	$ 171,955,108
Face Value	[2]	$ 172,000,000
U.S. Treasury Bills, 0.235% due July 7, 2016 [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity		25.39%
Fair Value		$ 207,872,080
Face Value		$ 208,000,000
United States Treasury Obligations [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity		93.89%	[3]	95.09%	[4],[5]
Fair Value		$ 768,801,792	[3]	$ 1,077,913,905	[4],[5]
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				13.67%
Fair Value				$ 154,993,645
Face Value				$ 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] $32,903,175 and $34,998,099, respectively is restricted for maintenance margin purposes.

[2] All or a portion of United States Treasury Obligations are on deposit with the Commodity Broker and held as maintenance margin for open futures contracts.

[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 period end.

[4] A portion of United States Treasury Obligations are on deposit with the Commodity Broker and held as maintenance margin for open futures contracts.

[5] 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.