Net Asset Value and Financial Highlights - Schedule of Net Asset Value Per Share (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Net Asset Value
Net asset value per Share, beginning of period	$ 29.27	$ 29.13	$ 29.16	$ 27.99
Net realized and change in unrealized gain (loss) on United States Treasury Obligations and Futures	$ 1.51	$ (1.86)	$ 1.67	$ (0.67)
Net investment income (loss)	$ (0.06)	$ (0.05)	$ (0.11)	$ (0.10)
Net income (loss)	$ 1.45	$ (1.91)	$ 1.56	$ (0.77)
Net asset value per Share, end of period	$ 30.72	$ 27.22	$ 30.72	$ 27.22
Market value per Share, beginning of period	$ 29.27	$ 29.11	$ 29.15	$ 27.94
Market value per Share, end of period	$ 30.75	$ 27.17	$ 30.75	$ 27.17
Ratio to average Net Assets
Net investment income (loss)	(0.74%)	(0.73%)	(0.73%)	(0.72%)
Total expenses	0.78%	0.79%	0.77%	0.79%
Total Return, at net asset value	4.95%	(6.56%)	5.35%	(2.75%)
Total Return, at market value	5.06%	(6.66%)	5.49%	(2.76%)