Condensed Schedule of Investments (Parenthetical)	3 Months Ended
	Mar. 31, 2013
Swap transaction as a percentage of total asset	20.24%
Foreign [Member] | ICE Natural Gas LD1 H Cleared Swap May 2013 contracts, expiring April 2013 [Member] | Open Cleared Swap and Futures Contracts [Member] | Open Cleared Swap Contracts, Long [Member]
Expiration date	April 2013
United States [Member] | NYMEX Natural Gas Futures NG May 2013 contracts, expiring April 2013 [Member] | Open Cleared Swap and Futures Contracts [Member] | Open Futures Contracts, Long [Member]
Expiration date	April 2013
United States [Member] | NYMEX Natural Gas Futures NN May 2013 contracts, expiring April 2013 [Member] | Open Cleared Swap and Futures Contracts [Member] | Open Futures Contracts, Long [Member]
Expiration date	April 2013
United States [Member] | U.S. Treasury Bills, 0.05%, 5/02/2013 [Member] | US Treasury Securities [Member] | Cash Equivalents [Member]
Interest rate	0.05%	[1]
Expiration date	5/02/2013	[1]
United States [Member] | U.S. Treasury Bills,0.08%, 5/30/2013 [Member] | US Treasury Securities [Member] | Cash Equivalents [Member]
Interest rate	0.08%
Expiration date	5/30/2013

[1] Security or partial security segregated as collateral for open over-the-counter total return swap contracts.