Schedule Of Investments (Treasury Bills) (USD $)	Dec. 31, 2012	Dec. 31, 2011
U.S. Treasury Bills, 0.06% due January 3, 2013 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	1.13%
Fair Value	$ 4,000,000
Face Value	4,000,000
U.S. Treasury Bills, 0.05% due January 10, 2013 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	3.39%
Fair Value	11,999,928
Face Value	12,000,000
U.S. Treasury Bills, 0.015% due January 17, 2013 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	3.25%
Fair Value	11,499,920
Face Value	11,500,000
U.S. Treasury Bills, 0.045% due January 24, 2013 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	59.88%
Fair Value	211,997,668
Face Value	212,000,000
U.S. Treasury Bills, 0.075% due January 31, 2013 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	2.82%
Fair Value	9,999,800
Face Value	10,000,000
U.S. Treasury Bills, 0.105% due February 14, 2013 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	2.54%
Fair Value	8,999,703
Face Value	9,000,000
U.S. Treasury Bills, 0.1% due February 28, 2013 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	1.13%
Fair Value	3,999,840
Face Value	4,000,000
U.S. Treasury Bills, 0.09% due March 7, 2013 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	0.85%
Fair Value	2,999,841
Face Value	3,000,000
U.S. Treasury Bills, 0.09% due March 14, 2013 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	1.13%
Fair Value	3,999,744
Face Value	4,000,000
U.S. Treasury Bills, 0.085% due March 28, 2013 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	2.26%
Fair Value	7,999,384
Face Value	8,000,000
Total United States Treasury Obligations [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	78.38%	96.33%
Fair Value	277,495,828	437,494,520
U.S. Treasury Bills, 0.015% Due January 5, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		5.73%
Fair Value		25,999,974
Face Value		26,000,000
U.S. Treasury Bills, 0.025% Due January 19, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		2.53%
Fair Value		11,499,943
Face Value		11,500,000
U.S. Treasury Bills, 0.015% Due January 26, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		60.33%
Fair Value		273,998,082
Face Value		274,000,000
U.S. Treasury Bills, 0.01% Due February 2, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		2.20%
Fair Value		9,999,910
Face Value		10,000,000
U.S. Treasury Bills, 0.01% Due February 16, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		5.06%
Fair Value		22,999,724
Face Value		23,000,000
U.S. Treasury Bills, 0.015% Due February 23, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		2.20%
Fair Value		9,999,820
Face Value		10,000,000
U.S. Treasury Bills, 0.03% due March 1, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		0.88%
Fair Value		3,999,900
Face Value		4,000,000
U.S. Treasury Bills, 0.005% Due March 8, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		0.66%
Fair Value		2,999,904
Face Value		3,000,000
U.S. Treasury Bills, 0.01% Due March 15, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		6.39%
Fair Value		28,998,985
Face Value		29,000,000
U.S. Treasury Bills, 0.005% Due March 22, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		6.17%
Fair Value		27,999,076
Face Value		28,000,000
U.S. Treasury Bills, 0.025% Due March 29, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		4.18%
Fair Value		18,999,202
Face Value		$ 19,000,000