Debt (Details)	9 Months Ended	12 Months Ended		9 Months Ended			9 Months Ended			9 Months Ended		9 Months Ended
	Sep. 30, 2013 USD ($)	Dec. 31, 2012 USD ($)	Sep. 30, 2013 Unsecured Debt Senior unsecured notes USD ($)	Sep. 30, 2013 Unsecured Debt Senior unsecured notes 5.30% to 7.18% USD ($)	Oct. 02, 2013 Unsecured Debt Senior unsecured notes 2.375% due October 2020 USD ($)	Oct. 02, 2013 Unsecured Debt Senior unsecured notes 2.375% due October 2020 EUR (€)	Sep. 30, 2013 Unsecured Debt Credit Facility and the Supplemental Facility USD ($) Item	Sep. 30, 2013 Unsecured Debt Credit Facility USD ($)	May 16, 2013 Unsecured Debt Credit Facility	Sep. 30, 2013 Unsecured Debt Supplemental Facility USD ($)	May 16, 2013 Unsecured Debt Supplemental Facility	Sep. 30, 2013 Unsecured Debt Term loan USD ($) Item	Sep. 30, 2013 Secured Debt Mortgages USD ($) note mortgagepool property	Dec. 31, 2012 Secured Debt Mortgages USD ($)
Debt
Total Mortgages and Other Indebtedness	$ 22,729,654,000	$ 23,113,007,000	$ 12,900,000,000									$ 240,000,000	$ 8,100,000,000	$ 8,000,000,000
Credit facility, amount outstanding								1,200,000,000		227,600,000
Maximum borrowing capacity								4,000,000,000		2,000,000,000		300,000,000
Available borrowing capacity							4,500,000,000
Maximum amount outstanding during period							1,600,000,000
Credit facility, weighted average amount outstanding							1,400,000,000
Letters of credit outstanding							45,000,000
Number of credit facilities							2
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				504,500,000									240,000,000
Interest rate, low end of range (as a percent)				5.30%
Interest rate, high end of range (as a percent)				7.18%
Number of one-year extension options available												2
Debt instrument extension period												1 year
Debt issued					1,000,000,000	750,000,000
Interest rate on debt (as a percent)					2.375%	2.375%
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
Fair value of debt
Carrying value of fixed-rate mortgages and unsecured indebtedness	20,700,000,000	21,000,000,000
Fair value of fixed-rate mortgages and unsecured indebtedness	$ 22,140,000,000	$ 23,373,000,000
Weighted average discount rates assumed in calculation of fair value for fixed-rate mortgages (as a percent)	3.55%	3.24%