Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013	Dec. 31, 2012
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Payable for securities purchased	$ 30,997,257		$ 30,997,257		$ 30,994,907
Cash held with Commodity Broker	3,591,743	13,448,558	3,591,743	13,448,558	3,113,981	22,027,189
Cash equivalents held	0		0		0
Average notional market value	340,400,000	228,600,000	307,900,000	197,600,000
Average charges paid to broker per round-turn trade, maximum	10.00	10.00	10.00	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	31,000,000		31,000,000		31,000,000
Payable for securities purchased	30,997,257		30,997,257		30,994,907
Level 2 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Assets and liabilities measured at fair value	0		0		0
Level 3 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Assets and liabilities measured at fair value	$ 0		$ 0		$ 0