Summary Of Significant Accounting Policies (Detail) (USD $)	9 Months Ended	12 Months Ended		3 Months Ended		9 Months Ended
	Sep. 30, 2012 M	Dec. 31, 2011	Sep. 30, 2011	Sep. 30, 2012 Maximum [Member]	Sep. 30, 2011 Maximum [Member]	Sep. 30, 2012 Maximum [Member]	Sep. 30, 2011 Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Restricted investments	$ 118,417,674	$ 148,081,288
Notional amount of United States Treasury Obligations unpaid	11,999,926	12,000,000
Cash and cash equivalents, original maturities, maximum number of months	3
Cash held by the Commodity Broker	114,201,676	210,907,413	196,748,360
Net unrealized appreciation (depreciation) on futures contracts	175,469,983	(135,420,844)
Management fee percentage per annum of daily net asset	0.85%
Average charges paid to broker per round-turn trade, maximum				$ 10.00	$ 10.00	$ 10.00	$ 10.00