Borrowings Under Line of Credit Arrangements and Related Items (Details Textual)	1 Months Ended	6 Months Ended
	Jul. 25, 2014 USD ($) properties Number	Jun. 30, 2014 USD ($) Number	Jun. 30, 2014 CAD	Jun. 30, 2014 GBP (£)	Dec. 31, 2013 USD ($)	Jun. 30, 2013 USD ($)
Line Of Credit Facility [Line Items]
Unsecured line of credit arrangement	$ 2,500,000,000	$ 2,250,000,000
Additional unsecured line of credit arrangement	1,000,000,000	1,000,000,000
Maximum aggregate commitment		3,250,000,000
Number of banks in consortium	28	30	30	30
Maximum Interest payable period in months		3	3	3
Available to borrow in alternate currencies	500,000,000	500,000,000
Agent bank's prime rate of interest		1.32%	1.32%	1.32%
Interest Rate margin	1.05%	1.175%	1.175%	1.175%
Annual facility fee for each bank based on commitment amount	0.20%	0.225%	0.225%	0.225%
Debt instrument maturity date	Oct. 31, 2018	Mar. 31, 2017	Mar. 31, 2017	Mar. 31, 2017
Balance outstanding at quarter end		$ 0	0	£ 0	$ 130,000,000	$ 0