Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Derivative Assets	$ 30,600
Financial Assets And Liabilities Not Measured At Fair Value By Balance Sheet Grouping [Line Items]
Mezzanine loans receivable	133,948	202,412
Notes and mortgages payable	8,558,275	8,255,101
Senior unsecured notes	1,357,661	1,082,928
Exchangeable senior debentures	497,898	491,000
Convertible senior debentures	10,168	186,413
Revolving credit facility debt	138,000	874,000
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Financial Assets And Liabilities Not Measured At Fair Value By Balance Sheet Grouping [Line Items]
Mezzanine loans receivable	133,948	202,412
Notes and mortgages payable	8,558,275	8,255,101
Senior unsecured notes	1,357,661	1,082,928
Exchangeable senior debentures	497,898	491,000
Convertible senior debentures	10,168	186,413
Revolving credit facility debt	138,000	874,000
Total debt	10,562,002	10,889,442
Estimate of Fair Value, Fair Value Disclosure [Member]
Financial Assets And Liabilities Not Measured At Fair Value By Balance Sheet Grouping [Line Items]
Mezzanine loans receivable	128,581	197,581
Notes and mortgages payable	8,685,619	8,446,791
Senior unsecured notes	1,426,406	1,119,512
Exchangeable senior debentures	509,982	554,355
Convertible senior debentures	10,220	191,510
Revolving credit facility debt	138,000	874,000
Total debt	10,770,227	11,186,168
Real Estate Fund [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance - changes in Level 3 " deferred compensation plan assets"	346,650	144,423	0
Fair Value Asset Measured on Recurring Basis, Asset, Purchases	248,803	144,423
Fair Value Asset Measured On Recurring Basis Asset Sales	(48,355)	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	17,386	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Other Net	(15,607)	0
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"	56,221	47,850	39,589
Fair Value Asset Measured on Recurring Basis, Asset, Purchases	25,692	17,006
Fair Value Asset Measured On Recurring Basis Asset Sales	(18,801)	(12,320)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	1,232	3,527
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Other Net	248	48
Fair Value, Measurements, Recurring [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Marketable Securities	741,321	766,116
Derivative Assets		17,616
Deferred Compensation Plan Assets	95,457	91,549
Real Estate Fund investments	346,650	144,423
Total assets	1,214,028	1,019,704
Mandatorily redeemable instruments (included in other liabilities)	54,865	55,097
Fair Value, Measurements, Nonrecurring [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Investments in Partially Owned Entities at Fair Value		11,413
Real Estate, Net at Fair Value	62,033	381,889
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Marketable Securities	741,321	766,116
Derivative Assets	0	0
Deferred Compensation Plan Assets	39,236	43,699
Real Estate Fund investments	0	0
Total assets	780,557	809,815
Mandatorily redeemable instruments (included in other liabilities)	54,865	55,097
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Investments in Partially Owned Entities at Fair Value		0
Real Estate, Net at Fair Value	0	0
Fair Value Inputs Level 2 Member | Fair Value, Measurements, Recurring [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Marketable Securities	0	0
Derivative Assets	30,600	17,616
Deferred Compensation Plan Assets	0	0
Real Estate Fund investments	0	0
Total assets	30,600	17,616
Mandatorily redeemable instruments (included in other liabilities)	0	0
Fair Value Inputs Level 2 Member | Fair Value, Measurements, Nonrecurring [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Investments in Partially Owned Entities at Fair Value		0
Real Estate, Net at Fair Value	0	0
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Marketable Securities	0	0
Derivative Assets	0	0
Deferred Compensation Plan Assets	56,221	47,850
Real Estate Fund investments	346,650	144,423
Total assets	402,871	192,273
Mandatorily redeemable instruments (included in other liabilities)	0	0
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Investments in Partially Owned Entities at Fair Value		11,413
Real Estate, Net at Fair Value	$ 62,033	$ 381,889