Income per Common Share of the Trust - Share Repurchase (Details) - Common shares [Member] - 2015 Share Repurchase Program [Member] $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2017 USD ($) shares	Sep. 30, 2017 USD ($) shares
Class of Stock [Line Items]
Authorized amount of shares available to be repurchased under a share repurchase plan | $	$ 250	$ 250
Number of shares repurchased during period | shares	0	0