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, 2015 USD ($) shares	Sep. 30, 2015 USD ($) shares
Class of Stock [Line Items]
Authorized amount of shares available to be repurchased under a share repurchase plan	$ 250.0	$ 250.0
Number of shares repurchased during period | shares	2,125,000	2,125,000
Amount paid for repurchase of shares	$ 65.5	$ 65.5