Income per Common Share of the Trust Share Repurchase (Details) - 2017 Share Repurchase Program [Member] - Common shares [Member] $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2018 USD ($) shares	Jun. 30, 2018 USD ($) shares
Equity, Class of Treasury Stock [Line Items]
Stock Repurchase Program, Authorized Amount | $	$ 250	$ 250
Stock Repurchased During Period, Shares | shares	0	0