Financial Highlights (Net Asset Value Based Total Return For A Shareholder) (Details)		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Investment Company, Financial Highlights [Abstract]
Total Return	[1]	3.78%	0.82%	8.24%	3.32%

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