Unaudited Schedule of Investments - Treasury (USD $)	Jun. 30, 2014	Dec. 31, 2013
Schedule of Investments [Line Items]
Fair Value	$ 374,990,428	$ 282,986,507
U.S. Treasury Bills, 0.035% due July 3, 2014 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	8.41%
Fair Value	31,000,000
Face Value	31,000,000
U.S. Treasury Bills, 0.030% due July 10, 2014 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	2.98%
Fair Value	11,000,000
Face Value	11,000,000
U.S. Treasury Bills, 0.030% due July 17, 2014 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	1.63%
Fair Value	5,999,976
Face Value	6,000,000
U.S. Treasury Bills, 0.010% due July 24, 2014 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	13.57%
Fair Value	49,999,550
Face Value	50,000,000
U.S. Treasury Bills, 0.025% due August 7, 2014 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	7.87%
Fair Value	28,999,971
Face Value	29,000,000
U.S. Treasury Bills, 0.025% due August 14, 2014 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	22.79%
Fair Value	83,998,236
Face Value	84,000,000
U.S. Treasury Bills, 0.025% due August 21, 2014 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	0.54%
Fair Value	1,999,946
Face Value	2,000,000
U.S. Treasury Bills, 0.030% due August 28, 2014 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	2.71%
Fair Value	9,999,740
Face Value	10,000,000
U.S. Treasury Bills, 0.035% due September 4, 2014 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	6.51%
Fair Value	23,999,400
Face Value	24,000,000
U.S. Treasury Bills, 0.035% due September 11, 2014 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	18.72%
Fair Value	68,997,930
Face Value	69,000,000
U.S. Treasury Bills, 0.035% due September 18, 2014 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	2.44%
Fair Value	8,999,676
Face Value	9,000,000
U.S. Treasury Bills, 0.025% due September 25, 2014 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	5.15%
Fair Value	18,998,917
Face Value	19,000,000
U.S. Treasury Bills, 0.040% due October 2, 2014 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	8.41%
Fair Value	30,997,086
Face Value	31,000,000
United States Treasury Obligations [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	101.73%	105.48%
Fair Value	374,990,428	282,986,507
Face Value
U.S. Treasury Bills, 0.030% due January 2, 2014 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		11.55%
Fair Value		31,000,000
Face Value		31,000,000
U.S. Treasury Bills, 0.025% due January 9, 2014 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		4.10%
Fair Value		10,999,978
Face Value		11,000,000
U.S. Treasury Bills, 0.010% due January 16, 2014 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		2.24%
Fair Value		5,999,982
Face Value		6,000,000
U.S. Treasury Bills, 0.005% due January 23, 2014 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		18.64%
Fair Value		49,999,650
Face Value		50,000,000
U.S. Treasury Bills, 0.005% due January 30, 2014 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		2.24%
Fair Value		5,999,940
Face Value		6,000,000
U.S. Treasury Bills, 0.050% due February 6, 2014 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		15.28%
Fair Value		40,999,631
Face Value		41,000,000
U.S. Treasury Bills, 0.075% due February 13, 2014 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		7.08%
Fair Value		18,999,829
Face Value		19,000,000
U.S. Treasury Bills, 0.080% due February 27, 2014 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		3.73%
Fair Value		9,999,690
Face Value		10,000,000
U.S. Treasury Bills, 0.075% due March 6, 2014 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		4.47%
Fair Value		11,999,004
Face Value		12,000,000
U.S. Treasury Bills, 0.070% due March 13, 2014 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		17.15%
Fair Value		45,996,872
Face Value		46,000,000
U.S. Treasury Bills, 0.065% due March 20, 2014 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		0.37%
Fair Value		999,882
Face Value		1,000,000
U.S. Treasury Bills, 0.070% due March 27, 2014 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		7.08%
Fair Value		18,997,226
Face Value		19,000,000
U.S. Treasury Bills, 0.065% due April 3, 2014 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		11.55%
Fair Value		30,994,823
Face Value		$ 31,000,000