Preferred Shares of Beneficial Interest (Preferred Dividends Declared Per Share) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
6.75% Series D Preferred Shares [Member]
Dividends Payable [Line Items]
Preferred shares of beneficial interest, dividend per share (in dollars per share)	$ 0.00	$ 0.00	$ 13.08
6.5% Series F Preferred Shares [Member]
Dividends Payable [Line Items]
Preferred shares of beneficial interest, dividend per share (in dollars per share)	$ 64.55	$ 162.50	$ 160.24