Financial Highlights (Net Asset Value Based Total Return For A Shareholder) (Details)		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Financial Highlights [Abstract]
Total Return	[1]	0.82%	0.82%	3.32%	(2.38%)

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