Financial Highlights (Expense Ratios To Average Equity) (Details)		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Financial Highlights [Abstract]
Operating expenses, before interest expense and other investment related expenses	[1],[2]	(2.83%)	(2.96%)	(2.83%)	(2.88%)
Interest expense and other investment related expenses	[1],[2]	(6.02%)	(3.76%)	(5.37%)	(3.31%)
Total Expenses	[1],[2]	(8.85%)	(6.72%)	(8.20%)	(6.19%)

[1] Average equity is calculated using month end values.

[2] Includes all items of income and expense on an annualized basis.