Fair Value Measurements (Details) (USD $) In Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Mar. 31, 2011	Dec. 31, 2010	Mar. 31, 2010	Dec. 31, 2009
Financial Assets And Liabilities Not Measured At Fair Value By Balance Sheet Grouping [Line Items]
Mezzanine loans receivable	$ 155,613		$ 155,613			$ 202,412
Notes and mortgages payable	8,575,022		8,575,022			8,259,298
Senior unsecured notes	982,629		982,629			1,082,928
Exchangeable senior debentures	494,403		494,403			491,000
Convertible senior debentures	187,994		187,994			186,413
Revolving credit facility debt	300,000		300,000			874,000
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"	255,795		255,795		230,657	144,423
Fair Value Asset Measured on Recurring Basis, Asset, Purchases	22,808		123,047
Fair Value Asset Measured On Recurring Basis Asset Sales	(12,831)		(12,831)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	15,957		16,655
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Other Net	(796)		(15,499)
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"	53,724	43,598	53,724	43,598	51,612	47,850	43,263	39,589
Fair Value Asset Measured on Recurring Basis, Asset, Purchases	17,818	3,210	19,104	6,342
Fair Value Asset Measured On Recurring Basis Asset Sales	(16,347)	(3,014)	(17,494)	(3,580)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	594	41	4,217	1,149
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Other Net	47	98	47	98
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Marketable Securities	791,676		791,676			766,116
Derivative Assets	0		0			0
Deferred Compensation Plan Assets	46,650		46,650			43,699
Real Estate Fund investments	0		0			0
Total assets	838,326		838,326			809,815
Mandatorily redeemable instruments (included in other liabilities)	55,097		55,097			55,097
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			0
Derivative Assets	28,017		28,017			17,616
Deferred Compensation Plan Assets	0		0			0
Real Estate Fund investments	0		0			0
Total assets	28,017		28,017			17,616
Mandatorily redeemable instruments (included in other liabilities)	0		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			0
Derivative Assets	0		0			0
Deferred Compensation Plan Assets	53,724		53,724			47,850
Real Estate Fund investments	255,795		255,795			144,423
Total assets	309,519		309,519			192,273
Mandatorily redeemable instruments (included in other liabilities)	0		0			0
Fair Value, Measurements, Recurring [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Marketable Securities	791,676		791,676			766,116
Derivative Assets	28,017		28,017			17,616
Deferred Compensation Plan Assets	100,374		100,374			91,549
Real Estate Fund investments	255,795		255,795			144,423
Total assets	1,175,862		1,175,862			1,019,704
Mandatorily redeemable instruments (included in other liabilities)	55,097		55,097			55,097
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	155,613		155,613			202,412
Notes and mortgages payable	8,575,022		8,575,022			8,259,298
Senior unsecured notes	982,629		982,629			1,082,928
Exchangeable senior debentures	494,403		494,403			491,000
Convertible senior debentures	187,994		187,994			186,413
Revolving credit facility debt	300,000		300,000			874,000
Total debt	10,540,048		10,540,048			10,893,639
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	149,948		149,948			197,581
Notes and mortgages payable	8,757,884		8,757,884			8,450,812
Senior unsecured notes	1,046,369		1,046,369			1,119,512
Exchangeable senior debentures	564,355		564,355			554,355
Convertible senior debentures	190,391		190,391			191,510
Revolving credit facility debt	300,000		300,000			874,000
Total debt	$ 10,858,999		$ 10,858,999			$ 11,190,189