Income per Common Share of the Trust (Units and Distributions) (Details) - Common shares [Member] - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Increase in shares of the Trust/units of general partner from the conversion of limited partners' units	9,826	9,044	14,491
Distributions per common share	$ 1.6	$ 1.90	$ 1.90