Debt (Details) (USD $)	6 Months Ended	12 Months Ended	6 Months Ended		6 Months Ended				6 Months Ended		6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 Mortgages	Jun. 30, 2013 Unsecured Debt Senior unsecured notes	Jun. 30, 2013 Unsecured Debt Senior unsecured notes 5.30% to 6.00%	Jun. 30, 2013 Unsecured Debt Credit Facility and the Supplemental Facility Item	Jun. 30, 2013 Unsecured Debt Credit Facility	May 16, 2013 Unsecured Debt Credit Facility	Jun. 30, 2013 Unsecured Debt Supplemental Facility	May 16, 2013 Unsecured Debt Supplemental Facility	Jun. 30, 2013 Unsecured Debt Term loan Item	Jun. 30, 2013 Mortgage Debt note property mortgagepool	Dec. 31, 2012 Mortgage Debt
Debt
Total Mortgages and Other Indebtedness	$ 22,687,622,000	$ 23,113,007,000		$ 13,000,000,000							$ 240,000,000	$ 8,000,000,000	$ 8,000,000,000
Credit facility, amount outstanding							1,200,000,000		226,100,000
Maximum borrowing capacity							4,000,000,000		2,000,000,000		300,000,000
Available borrowing capacity						4,600,000,000
Number of credit facilities						2
Maximum amount outstanding during period						1,600,000,000
Credit facility, weighted average amount outstanding						1,500,000,000
Letters of credit outstanding						45,100,000
Optional expanded maximum borrowing capacity							5,000,000,000		2,500,000,000
Reference rate							LIBOR		LIBOR		LIBOR
Interest added to reference rate (as a percent)							0.95%		0.95%		1.10%
Additional facility fee (as a percent)							0.15%		0.15%
Reduction in interest added to reference rate (as a percent)								0.05%		0.05%
Multi-currency tranche							2,000,000,000
Amount of debt redeemed			240,000,000		429,500,000
Interest rate, low end of range (as a percent)					5.30%
Interest rate, high end of range (as a percent)					6.00%
Debt covenants
Number of non-recourse mortgage notes under which the Company and subsidiaries are borrowers												79
Number of properties secured by non-recourse mortgage notes												79
Number of cross-defaulted and cross-collateralized mortgage pools												7
Total number of properties pledged as collateral for cross defaulted and cross collateralized mortgages												27
Number of one-year extension options available											2
Debt instrument extension period											1 year
Fair value of debt
Carrying value of fixed-rate mortgages and unsecured indebtedness	20,900,000,000	21,000,000,000
Fair value of fixed-rate mortgages and unsecured indebtedness	$ 22,396,000,000	$ 23,373,000,000
Weighted average discount rates assumed in calculation of fair value for fixed-rate mortgages (as a percent)	3.68%	3.24%