Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Unobservable Quantitative Input [Abstract]
Real Estate Fund Nondepreciable Real Estate	$ 600,786	$ 346,650
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage and mezzanine loans receivable	225,359	133,948
Mortgages payable	8,768,182	8,072,880
Senior unsecured notes	1,358,008	1,357,661
Exchangeable senior debentures	0	497,898
Convertible senior debentures	0	10,168
Revolving credit facility debt	1,170,000	138,000
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	543,000	0
Mortgage and mezzanine loans receivable	225,359	133,948
Total assets	768,359	133,948
Mortgages payable	8,768,182	8,072,880
Senior unsecured notes	1,358,008	1,357,661
Exchangeable senior debentures	0	497,898
Convertible senior debentures	0	10,168
Revolving credit facility debt	1,170,000	138,000
Total debt	11,296,190	10,076,607
Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash equivalents	543,000	0
Mortgage and mezzanine loans receivable	221,446	128,581
Total assets	764,446	128,581
Mortgages payable	8,795,000	8,188,000
Senior unsecured notes	1,468,000	1,426,000
Exchangeable senior debentures	0	510,000
Convertible senior debentures	0	10,000
Revolving credit facility debt	1,170,000	138,000
Total debt	11,433,000	10,272,000
Real Estate Funds [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance - changes in Level 3 " deferred compensation plan assets"	600,786	346,650	144,423
Purchases	262,251	248,803
Sales	(63,762)	(48,355)
Realized gains	0	5,391
Unrealized gains	55,361	11,995
Other, Net	286	(15,607)
Real Estate Funds [Member] | Minimum [Member]
Unobservable Quantitative Input [Abstract]
Discount rates	12.50%
Terminal capitalization rates	5.30%
Real Estate Funds [Member] | Maximum [Member]
Unobservable Quantitative Input [Abstract]
Discount rates	19.00%
Terminal capitalization rates	6.30%
Real Estate Funds [Member] | Weighted Average [Member]
Unobservable Quantitative Input [Abstract]
Discount rates	14.70%
Terminal capitalization rates	5.80%
Deferred Compensation Plan Asset [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance - changes in Level 3 " deferred compensation plan assets"	62,631	56,221	47,850
Purchases	9,951	25,692
Sales	(8,367)	(18,801)
Realized and unrealized gains	4,703	1,232
Other, Net	123	248
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities	398,188	741,321
Derivative Assets	11,165	30,600
Deferred Compensation Plan Assets	105,200	95,457
Real Estate Fund investments	600,786	346,650
Total assets	1,115,339	1,214,028
Mandatorily redeemable instruments (included in other liabilities)	55,011	54,865
Interest rate swap (included in other liabilities)	50,070	41,114
Total liabilities	105,081	95,979
Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in Partially Owned Entities at Fair Value	478,041
Real Estate, Net at Fair Value	189,529	62,033
Condominium units (included in other assets)	52,142
Total Assets, Nonrecurring	719,712
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities	398,188	741,321
Derivative Assets	0	0
Deferred Compensation Plan Assets	42,569	39,236
Real Estate Fund investments	0	0
Total assets	440,757	780,557
Mandatorily redeemable instruments (included in other liabilities)	55,011	54,865
Interest rate swap (included in other liabilities)	0	0
Total liabilities	55,011	54,865
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in Partially Owned Entities at Fair Value
Real Estate, Net at Fair Value	0	0
Condominium units (included in other assets)	0
Total Assets, Nonrecurring	0
Fair Value Inputs Level 2 Member | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities	0	0
Derivative Assets	11,165	30,600
Deferred Compensation Plan Assets	0	0
Real Estate Fund investments	0	0
Total assets	11,165	30,600
Mandatorily redeemable instruments (included in other liabilities)	0	0
Interest rate swap (included in other liabilities)	50,070	41,114
Total liabilities	50,070	41,114
Fair Value Inputs Level 2 Member | Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in Partially Owned Entities at Fair Value
Real Estate, Net at Fair Value	0	0
Condominium units (included in other assets)	0
Total Assets, Nonrecurring	0
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable Securities	0	0
Derivative Assets	0	0
Deferred Compensation Plan Assets	62,631	56,221
Real Estate Fund investments	600,786	346,650
Total assets	663,417	402,871
Mandatorily redeemable instruments (included in other liabilities)	0	0
Interest rate swap (included in other liabilities)	0	0
Total liabilities	0	0
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in Partially Owned Entities at Fair Value	478,041
Real Estate, Net at Fair Value	189,529	62,033
Condominium units (included in other assets)	52,142
Total Assets, Nonrecurring	$ 719,712