Debt (Details) (USD $)	6 Months Ended		12 Months Ended	6 Months Ended			6 Months Ended		1 Months Ended		1 Months Ended	3 Months Ended	1 Months Ended		1 Months Ended		1 Months Ended		1 Months Ended	3 Months Ended		1 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010	Jun. 30, 2011 Secured Debt	Dec. 31, 2010 Secured Debt	Jun. 30, 2011 Unsecured Debt Senior unsecured notes	Jun. 30, 2011 Unsecured Debt Credit Facility	Jun. 30, 2011 Mortgage Covenants	Jan. 31, 2010 Senior unsecured notes tendered January 12, 2010	Jan. 26, 2010 Senior unsecured notes tendered January 12, 2010	Jan. 31, 2010 Senior unsecured notes issue closed January 25, 2010	Mar. 31, 2010 Senior unsecured notes issue closed January 25, 2010	Jan. 31, 2010 Senior unsecured notes 4.20% due 2015	Jan. 25, 2010 Senior unsecured notes 4.20% due 2015	Jan. 31, 2010 Senior unsecured notes 5.65% due 2020	Jan. 25, 2010 Senior unsecured notes 5.65% due 2020	Jan. 31, 2010 Senior unsecured notes 6.75% due 2040	Jan. 25, 2010 Senior unsecured notes 6.75% due 2040	Aug. 31, 2010 Senior unsecured notes issue closed August 16, 2010	Sep. 30, 2010 Senior unsecured notes issue closed August 16, 2010	Aug. 16, 2010 Senior unsecured notes issue closed August 16, 2010	Aug. 31, 2010 Senior unsecured notes tendered August 9, 2010	Aug. 17, 2010 Senior unsecured notes tendered August 9, 2010
Debt
Total Mortgages and Other Indebtedness	$ 17,013,893,000		$ 17,473,760,000	$ 6,500,000,000	$ 6,600,000,000	$ 9,600,000,000
Credit facility, amount outstanding							860,000,000
Credit facility, maximum borrowing capacity							3,900,000,000
Line of credit facility, expanded maximum borrowing capacity in terms of accordion feature							4,000,000,000
Reference rate							LIBOR
Interest added to reference rate (in basis points)							210
Facility fee (in basis points)							40
Line of credit facility, competitive bid feature maximum bid (as a percent)							50.00%
Credit facility, maximum amount outstanding							860,400,000
Foreign currency denominated credit facilities outstanding							275,000,000
Credit facility, weighted average amount outstanding							857,600,000
Letters of credit outstanding							36,000,000
Number of notes included in the bond tender offer									10													3
Principal amount of bond tender offer										2,300,000,000													1,330,000,000
Debt instrument weighted average duration (in years)									2.0		14.4											3.5
Debt instrument weighted average interest rate (as a percent)				6.99%					5.76%		5.69%											6.06%
Proceeds from issuance of senior unsecured notes	205,946,000	2,296,533,000									2,250,000,000		400,000,000		1,250,000,000		600,000,000		900,000,000
Interest rate on debt (as a percent)	7.00%													4.20%		5.65%		6.75%			4.375%
Charge recorded in earnings on repayment of debt		165,625,000										165,600,000								185,100,000
Senior unsecured note repayment	382,800,000
Interest rate, low end of range (as a percent)	5.38%
Interest rate, high end of range (as a percent)	8.25%
Carrying value of fixed-rate mortgages and other indebtedness	14,300,000,000		14,800,000,000
Fair value of fixed-rate mortgages and other indebtedness	15,722,000,000		16,087,000,000
Weighted average discount rates assumed in calculation of fair value for fixed-rate mortgages (as a percent)	4.23%		4.46%
Number of unencumbered properties on repayment of debt				2
Repayment of mortgage loans				$ 120,600,000
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								11
Total number of properties pledged as collateral for cross defaulted and cross collateralized mortgages								48