Income per Common Share of the Trust - Share Repurchase (Details) - Common shares [Member] - 2015 Share Repurchase Program [Member] $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2016 USD ($) $ / shares shares
Class of Stock [Line Items]
Authorized amount of shares available to be repurchased under a share repurchase plan	$ 250.0
Number of shares repurchased during period | shares	1.4
Amount paid for repurchase of shares	$ 40.9
Treasury Stock Acquired, Average Cost Per Share | $ / shares	$ 29.28