Preferred Shares of Beneficial Interest (Preferred Dividends Declared Per Share) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
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