Financial Highlights (Net Asset Value Based Total Return For A Shareholder) (Details)		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Investment Company, Financial Highlights [Abstract]
Total Return	[1]	1.05%	0.13%	4.40%	(2.25%)

[1] Total return is calculated assuming reinvestment of distributions at shareholders' equity per share during the period.