Summary Of Significant Accounting Policies (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012 M	Dec. 31, 2011 M	Dec. 31, 2010
Summary Of Significant Accounting Policies [Line Items]
Percentage of limited units purchased by fund	100.00%
Restricted investments	$ 18,351,850	$ 42,518,250
Notional Amount		11,000,000
Amount of securities sold		10,999,933
Cash held by Commodity Broker	85,390,111	65,165,354
Deposit to satisfy margin on future contracts	18,351,850	48,362,065
Cash and cash equivalents, original maturities, maximum number of months
Net unrealized appreciation (depreciation) on futures contracts	8,628,985	48,362,065
Management fee percentage per annum of daily net asset		0.75%
Average charges paid to Broker per round-turn trade, maximum	$ 10.00	$ 10.00	$ 10.00