Financial Highlights (Expense Ratios To Average Equity) (Details)		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investment Company, Financial Highlights [Abstract]
Operating expenses, before interest expense and other investment related expenses	[1]	(2.81%)	(2.93%)	(2.69%)
Interest expense and other investment related expenses	[1]	(6.41%)	(3.56%)	(2.30%)
Total Expenses	[1]	(9.22%)	(6.49%)	(4.99%)

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