Financial Highlights (Net Asset Value Based Total Return For A Shareholder) (Details)		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investment Company, Financial Highlights [Abstract]
Total Return	[1]	6.14%	(1.83%)	5.14%

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