Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
LME Commodity Futures Contracts Transferred from Level 2 to Level 1	$ (9,353,669)		$ (9,353,669)
Payable for securities purchased					$ 22,997,987
Cash and cash equivalents, original maturities, maximum number of months			3 months
Payable to the Commodity Broker for LME Contract	16,313,530		$ 16,313,530		7,104,250
Average charges paid to broker per round-turn trade, maximum	19.00	$ 10.00	19.00	$ 10.00
US Treasury Securities Purchased But Not Yet Paid [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Face amount of debt purchased					23,000,000
Payable for securities purchased	0		0		$ 22,997,987
US Treasury Securities Sold But Not Yet Received [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Receivable for securities sold	$ 0		$ 0