Financial Instruments (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Investment Holdings [Line Items]
Net unrealized gains (losses) on open contracts	$ (1,351,338)	$ (1,105,501)
Cash held by acting commodity futures brokers administrating open contracts	43,681,397	43,297,414
Exchange-Traded [Member]
Investment Holdings [Line Items]
Net unrealized gains (losses) on open contracts	(1,170,739)	(2,010,679)
Longest maturity date	Sep. 30, 2014	Sep. 30, 2014
Off-Exchange-Traded [Member]
Investment Holdings [Line Items]
Net unrealized gains (losses) on open contracts	$ (180,599)	$ 905,178
Longest maturity date	Jun. 30, 2013	Mar. 31, 2013