Statements of Changes in Shareholders' Capital (Unaudited) (USD $)	3 Months Ended	6 Months Ended		12 Months Ended
	Jun. 30, 2012	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Statements of Changes in Shareholders' Capital [Abstract]
Shareholders' capital-beginning of period		$ 214,180,129	$ 247,757,748	$ 247,757,748
Repurchase of shares		(203,766)		(748,433)
Net increase (decrease) in shareholders' capital resulting from operations:
Net investment income (loss)	(840,928)	(1,696,511)	(1,824,640)	(3,612,955)
Net realized gain (loss) from:
Short-term investments		(652)		21,761
Futures contracts		(16,939,576)		(2,120,418)
Call options written		4,236,174		10,035,572
Change in net unrealized appreciation (depreciation) of:
Short-term investments	(4,822)	(28,176)	(7,586)	4,506
Futures contracts	7,190,243	7,710,570	(22,956,284)	(23,776,469)
Call options written	(1,352,546)	(1,709,545)	2,364,079	2,741,582
Net income (loss)	(15,574,687)	(8,427,716)	974,460	(16,706,421)
Distributions to shareholders		(8,020,740)		(16,122,765)
Shareholders' capital-end of period	$ 197,527,907	$ 197,527,907		$ 214,180,129
Shares-beginning of period		9,229,040	9,267,040	9,267,040
Issuance of shares		0		0
Repurchase of shares		(9,800)		(38,000)
Shares-end of period	9,219,240	9,219,240		9,229,040