Schedule Of Investments (Treasury) (Detail) (USD $)	Sep. 30, 2012	Dec. 31, 2011
Schedule of Investments [Line Items]
United States Treasury Obligations, cost	$ 1,629,756,268	$ 1,959,877,867
U.S. Treasury Bills, 0.105% Due October 4, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	23.79%
Fair Value	455,998,176
Face Value	456,000,000
U.S. Treasury Bills, 0.09% Due October 11, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	2.24%
Fair Value	42,999,441
Face Value	43,000,000
U.S. Treasury Bills, 0.08% Due October 18, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	2.08%
Fair Value	39,898,803
Face Value	39,900,000
U.S. Treasury Bills, 0.055% Due October 25, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	11.84%
Fair Value	226,991,601
Face Value	227,000,000
U.S. Treasury Bills, 0.1% Due November 1, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	14.82%
Fair Value	283,986,652
Face Value	284,000,000
U.S. Treasury Bills, 0.1% Due November 8, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	2.77%
Fair Value	52,996,078
Face Value	53,000,000
U.S. Treasury Bills, 0.11% Due November 15, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	1.93%
Fair Value	36,996,781
Face Value	37,000,000
U.S. Treasury Bills, 0.105% Due November 23, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	6.52%
Fair Value	124,986,625
Face Value	125,000,000
U.S. Treasury Bills, 0.105% Due November 29, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	0.11%
Fair Value	1,999,804
Face Value	2,000,000
U.S. Treasury Bills, 0.1% Due December 6, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	11.69%
Fair Value	223,969,312
Face Value	224,000,000
U.S. Treasury Bills, 0.1% Due December 13, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	3.18%
Fair Value	60,990,118
Face Value	61,000,000
U.S. Treasury Bills, 0.105% Due December 20, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	1.67%
Fair Value	31,994,304
Face Value	32,000,000
U.S. Treasury Bills, 0.11% Due December 27, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets	2.40%
Fair Value	45,989,742
Face Value	46,000,000
Total United States Treasury Obligations [Member]
Schedule of Investments [Line Items]
United States Treasury Obligations, cost	1,629,756,268	1,959,877,867
Percentage of Net Assets	85.04%	96.34%
Fair Value	1,629,797,437	1,959,875,160
U.S. Treasury Bills, 0.015% Due January 5, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		27.48%
Fair Value		558,999,441
Face Value		559,000,000
U.S. Treasury Bills, 0.01% Due January 12, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		2.66%
Fair Value		53,999,838
Face Value		54,000,000
U.S. Treasury Bills, 0.025% Due January 19, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		0.19%
Fair Value		3,899,981
Face Value		3,900,000
U.S. Treasury Bills, 0.015% Due January 26, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		13.91%
Fair Value		282,998,019
Face Value		283,000,000
U.S. Treasury Bills, 0.01% Due February 2, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		18.78%
Fair Value		381,996,562
Face Value		382,000,000
U.S. Treasury Bills, 0.01% Due February 16, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		2.70%
Fair Value		54,999,340
Face Value		55,000,000
U.S. Treasury Bills, 0.015% Due February 23, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		7.47%
Fair Value		151,997,264
Face Value		152,000,000
U.S. Treasury Bills, 0.03% Due March 1, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		6.29%
Fair Value		127,996,800
Face Value		128,000,000
U.S. Treasury Bills, 0.005% Due March 8, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		7.23%
Fair Value		146,995,296
Face Value		147,000,000
U.S. Treasury Bills, 0.01% Due March 15, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		5.41%
Fair Value		109,996,150
Face Value		110,000,000
U.S. Treasury Bills, 0.005% Due March 22, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		0.44%
Fair Value		8,999,703
Face Value		9,000,000
U.S. Treasury Bills, 0.025% Due March 29, 2012 [Member]
Schedule of Investments [Line Items]
Percentage of Net Assets		3.78%
Fair Value		76,996,766
Face Value		$ 77,000,000