Net Asset Value And Financial Highlights (Schedule Of Net Asset Value Per Share) (Detail) (USD $)	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Net Asset Value
Net asset value per Share, beginning of period	$ 28.14		$ 32.00		$ 28.81		$ 32.37
Net realized and change in unrealized gain (loss) on United States Treasury Obligations and Futures	$ 1.41		$ (2.00)		$ 0.87		$ (2.22)
Net investment income (loss)	$ (0.06)		$ (0.07)		$ (0.19)		$ (0.22)
Net income (loss)	$ 1.35		$ (2.07)		$ 0.68		$ (2.44)
Net asset value per Share, end of period	$ 29.49		$ 29.93		$ 29.49		$ 29.93
Market value per Share, beginning of period	$ 28.22		$ 31.74		$ 28.88		$ 32.35
Market value per Share, end of period	$ 29.41		$ 29.67		$ 29.41		$ 29.67
Ratio to average Net Assets
Net investment income (loss)	(0.83%)	[1]	(0.87%)	[1]	(0.87%)	[1]	(0.84%)	[1]
Total expenses	0.91%	[1]	0.91%	[1]	0.93%	[1]	0.92%	[1]
Total Return, at net asset value	4.80%	[2]	(6.47%)	[2]	2.36%	[2]	(7.54%)	[2]
Total Return, at market value	4.22%	[2]	(6.52%)	[2]	1.84%	[2]	(8.28%)	[2]

[1] Percentages are annualized.

[2] Percentages are not annualized.