Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011	Dec. 31, 2010
Unobservable Quantitative Input [Abstract]
Real Estate Fund Nondepreciable Real Estate	$ 324,514		$ 346,650
Financial Assets And Liabilities Not Measured At Fair Value By Balance Sheet Grouping [Line Items]
Mezzanine loans receivable	133,143		133,948
Notes and mortgages payable	8,434,938		8,558,275
Senior unsecured notes	1,357,748		1,357,661
Exchangeable senior debentures	499,680		497,898
Convertible senior debentures	10,233		10,168
Revolving credit facility debt	0		138,000
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	133,143		133,948
Notes and mortgages payable	8,434,938		8,558,275
Senior unsecured notes	1,357,748		1,357,661
Exchangeable senior debentures	499,680		497,898
Convertible senior debentures	10,233		10,168
Revolving credit facility debt	0		138,000
Total Debt	10,302,599		10,562,002
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	128,000		129,000
Notes and mortgages payable	8,505,000		8,686,000
Senior unsecured notes	1,439,000		1,426,000
Exchangeable senior debentures	501,000		510,000
Convertible senior debentures	10,000		10,000
Revolving credit facility debt	0		138,000
Total Debt	10,455,000		10,770,000
Fair Value, Measurements, Recurring [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Marketable Securities	754,510		741,321
Derivative position in marketable equity securities	31,645		30,600
Deferred Compensation Plan Assets	99,810		95,457
Real Estate Fund investments	324,514		346,650
Total assets	1,210,479		1,214,028
Mandatorily redeemable instruments (included in other liabilities)	55,097		54,865
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Marketable Securities	754,510		741,321
Derivative position in marketable equity securities	0		0
Deferred Compensation Plan Assets	40,929		39,236
Real Estate Fund investments	0		0
Total assets	795,439		780,557
Mandatorily redeemable instruments (included in other liabilities)	55,097		54,865
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 position in marketable equity securities	31,645		30,600
Deferred Compensation Plan Assets	0		0
Real Estate Fund investments	0		0
Total assets	31,645		30,600
Mandatorily redeemable instruments (included in other liabilities)	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 position in marketable equity securities	0		0
Deferred Compensation Plan Assets	58,881		56,221
Real Estate Fund investments	324,514		346,650
Total assets	383,395		402,871
Mandatorily redeemable instruments (included in other liabilities)	0		0
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"	324,514	230,657	346,650	144,423
Fair Value Asset Measured on Recurring Basis, Asset, Purchases	0	100,238
Fair Value Asset Measured On Recurring Basis Asset Sales	(31,052)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	6,844	698
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Other Net	2,072	(14,702)
Unobservable Quantitative Input [Abstract]
Fair Value Measurements, Valuation Processes, Description	We use a discounted cash flow valuation technique to estimate the fair value of each of these investments, which is updated quarterly by personnel responsible for the management of each investment and reviewed by senior management at each reporting period. The discounted cash flow valuation technique requires us to estimate cash flows for each investment over the anticipated holding period, which currently ranges from 2.3 to 6.8 years. Cash flows are derived from property rental revenue (base rents plus reimbursements) less operating expenses, real estate taxes and capital and other costs, plus projected sales proceeds in the year of exit. Property rental revenue is based on leases currently in place and our estimates for future leasing activity, which are based on current market rents for similar space plus a projected growth factor. Similarly, estimated operating expenses and real estate taxes are based on amounts incurred in the current period plus a projected growth factor for future periods. Anticipated sales proceeds at the end of an investment’s expected holding period are determined based on the net cash flow of the investment in the year of exit, divided by a terminal capitalization rate, less estimated selling costs. The fair value of each property is calculated by discounting the future cash flows (including the projected sales proceeds), using an appropriate discount rate and then reduced by the property’s outstanding debt, if any, to determine the fair value of the equity in each investment. Significant unobservable quantitative inputs used in determining the fair value of each investment include capitalization rates and discount rates. These rates are based on the location, type and nature of each property, and current and anticipated market conditions, which are derived from original underwriting assumptions, industry publications and from the experience of our Acquisitions and Capital Markets departments.
Real Estate Fund [Member] | Minimum [Member]
Unobservable Quantitative Input [Abstract]
Fair Value Inputs, Discount Rate	12.50%
Fair Value Inputs, Cap Rate	5.50%
Fair Value Inputs, Holding Period	2 years 4 months
Real Estate Fund [Member] | Maximum [Member]
Unobservable Quantitative Input [Abstract]
Fair Value Inputs, Discount Rate	23.30%
Fair Value Inputs, Cap Rate	6.80%
Fair Value Inputs, Holding Period	6 years 9 months 18 days
Real Estate Fund [Member] | Weighted Average [Member]
Unobservable Quantitative Input [Abstract]
Fair Value Inputs, Discount Rate	15.00%
Fair Value Inputs, Cap Rate	5.90%
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"	58,881	51,612	56,221	47,850
Fair Value Asset Measured on Recurring Basis, Asset, Purchases	3,611	1,286
Fair Value Asset Measured On Recurring Basis Asset Sales	(3,395)	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	2,392	3,623
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Other Net	$ 52	$ (1,147)
Unobservable Quantitative Input [Abstract]
Fair Value Measurements, Valuation Processes, Description	Deferred compensation plan assets that are classified as Level 3 consist of investments in limited partnerships and investment funds, which are managed by third parties. We receive quarterly financial reports from a third-party administrator, which are compiled from the quarterly reports provided to them from each limited partnership and investment fund. The quarterly reports provide net asset values on a fair value basis which are audited by independent public accounting firms on an annual basis. The third-party administrator does not adjust these values in determining our share of the net assets and we do not adjust these values when reported in our consolidated financial statements.