Schedule Of Investments (Treasury Bills) (Parenthetical) (USD $)	12 Months Ended												12 Months Ended
	Dec. 31, 2012 U.S. Treasury Bills, 0.06% due January 3, 2013 [Member]	Dec. 31, 2012 U.S. Treasury Bills, 0.05% due January 10, 2013 [Member]	Dec. 31, 2012 U.S. Treasury Bills, 0.015% due January 17, 2013 [Member]	Dec. 31, 2012 U.S. Treasury Bills, 0.045% due January 24, 2013 [Member]	Dec. 31, 2012 U.S. Treasury Bills, 0.075% due January 31, 2013 [Member]	Dec. 31, 2012 U.S. Treasury Bills, 0.105% due February 14, 2013 [Member]	Dec. 31, 2012 U.S. Treasury Bills, 0.1% due February 28, 2013 [Member]	Dec. 31, 2012 U.S. Treasury Bills, 0.09% due March 7, 2013 [Member]	Dec. 31, 2012 U.S. Treasury Bills, 0.09% due March 14, 2013 [Member]	Dec. 31, 2012 U.S. Treasury Bills, 0.085% due March 28, 2013 [Member]	Dec. 31, 2012 Total United States Treasury Obligations [Member]	Dec. 31, 2011 Total United States Treasury Obligations [Member]	Dec. 31, 2011 U.S. Treasury Bills, 0.015% Due January 5, 2012 [Member]	Dec. 31, 2011 U.S. Treasury Bills, 0.025% Due January 19, 2012 [Member]	Dec. 31, 2011 U.S. Treasury Bills, 0.015% Due January 26, 2012 [Member]	Dec. 31, 2011 U.S. Treasury Bills, 0.01% Due February 2, 2012 [Member]	Dec. 31, 2011 U.S. Treasury Bills, 0.01% Due February 16, 2012 [Member]	Dec. 31, 2011 U.S. Treasury Bills, 0.015% Due February 23, 2012 [Member]	Dec. 31, 2011 U.S. Treasury Bills, 0.03% due March 1, 2012 [Member]	Dec. 31, 2011 U.S. Treasury Bills, 0.005% Due March 8, 2012 [Member]	Dec. 31, 2011 U.S. Treasury Bills, 0.01% Due March 15, 2012 [Member]	Dec. 31, 2011 U.S. Treasury Bills, 0.005% Due March 22, 2012 [Member]	Dec. 31, 2011 U.S. Treasury Bills, 0.025% Due March 29, 2012 [Member]
Schedule of Investments [Line Items]
Investment Interest Rate	0.06%	0.05%	0.015%	0.045%	0.075%	0.105%	0.10%	0.09%	0.09%	0.085%			0.015%	0.025%	0.015%	0.01%	0.01%	0.015%	0.03%	0.005%	0.01%	0.005%	0.025%
Investment Maturity Date	Jan. 03, 2013	Jan. 10, 2013	Jan. 17, 2013	Jan. 24, 2013	Jan. 31, 2013	Feb. 14, 2013	Feb. 28, 2013	Mar. 07, 2013	Mar. 14, 2013	Mar. 28, 2013			Jan. 05, 2012	Jan. 19, 2012	Jan. 26, 2012	Feb. 02, 2012	Feb. 16, 2012	Feb. 23, 2012	Mar. 01, 2012	Mar. 08, 2012	Mar. 15, 2012	Mar. 22, 2012	Mar. 29, 2012
Total United States Treasury Obligations (cost $277,480,954)											$ 277,480,954	$ 437,494,687