Debt (Details) (USD $)	6 Months Ended	12 Months Ended	6 Months Ended	1 Months Ended		1 Months Ended		1 Months Ended		3 Months Ended		6 Months Ended			3 Months Ended		6 Months Ended			6 Months Ended
	Jun. 30, 2012 item	Dec. 31, 2011	Jun. 30, 2012 Unsecured Debt Senior unsecured notes	Mar. 31, 2012 Unsecured Debt Senior unsecured notes 2.15%	Mar. 13, 2012 Unsecured Debt Senior unsecured notes 2.15%	Mar. 31, 2012 Unsecured Debt Senior unsecured notes 3.375%	Mar. 13, 2012 Unsecured Debt Senior unsecured notes 3.375%	Mar. 31, 2012 Unsecured Debt Senior unsecured notes 4.75%	Mar. 13, 2012 Unsecured Debt Senior unsecured notes 4.75%	Mar. 31, 2012 Unsecured Debt Term loan	Nov. 02, 2011 Unsecured Debt Term loan	Jun. 30, 2012 Unsecured Debt Credit Facility and the Supplemental Facility	Jun. 30, 2012 Unsecured Debt Credit Facility	Jun. 30, 2012 Unsecured Debt Credit Facility Euro	Jun. 30, 2012 Unsecured Debt Credit Facility Yen	Jun. 30, 2012 Unsecured Debt Supplemental Facility	Jun. 30, 2012 Unsecured Debt Supplemental Facility	Jun. 30, 2012 Secured Debt note property mortgagepool	Dec. 31, 2011 Secured Debt	Jun. 30, 2012 Secured Debt Term loan	Jun. 30, 2012 Secured Debt Mortgages property	Jun. 30, 2012 Secured Debt Mortgages The Mills acquisition property
Debt
Total Mortgages and Other Indebtedness	$ 22,466,558,000	$ 18,446,440,000	$ 12,300,000,000											$ 1,100,000,000				$ 8,000,000,000	$ 6,800,000,000			$ 2,600,000,000
Credit facility, amount outstanding													1,800,000,000			278,300,000	278,300,000
Maximum borrowing capacity											900,000,000		4,000,000,000			2,000,000,000	2,000,000,000
Available borrowing capacity												3,900,000,000
Number of credit facilities	2
Maximum amount outstanding during period												3,100,000,000
Credit facility, weighted average amount outstanding												1,600,000,000
Letters of credit outstanding													38,900,000
Optional expanded maximum borrowing capacity													5,000,000,000				2,500,000,000
Reference rate													LIBOR				LIBOR
Interest added to reference rate (as a percent)													1.00%			1.00%	1.00%
Additional facility fee (as a percent)													0.15%			0.15%	0.15%
Multi-currency tranche													2,000,000,000
Debt issued				600,000,000		600,000,000		550,000,000		160,000,000						285,000,000
Interest rate on debt (as a percent)					2.15%		3.375%		4.75%
Average interest rate (as a percent)																					3.17%
Amount of debt redeemed			124,900,000												285,000,000					735,000,000	427,800,000
Interest rate, low end of range (as a percent)			5.75%
Interest rate, high end of range (as a percent)			6.88%
Number of unencumbered properties on repayment of debt																					7
Number of properties encumbered by assumption of debt in acquisition																						9
Debt covenants
Number of non-recourse mortgage notes under which the Company and subsidiaries are borrowers																		90
Number of properties secured by non-recourse mortgage notes																		90
Number of cross-defaulted and cross-collateralized mortgage pools																		8
Total number of properties pledged as collateral for cross defaulted and cross collateralized mortgages																		38
Fair value of debt
Carrying value of fixed-rate mortgages and other indebtedness	19,800,000,000	15,900,000,000
Fair value of fixed-rate mortgages and other indebtedness	$ 22,065,000	$ 17,905,000
Weighted average discount rates assumed in calculation of fair value for fixed-rate mortgages (as a percent)	3.51%	3.60%