Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011	Dec. 31, 2010
Unobservable Quantitative Input [Abstract]
Real Estate Fund Nondepreciable Real Estate	$ 482,442		$ 482,442			$ 346,650
Unrealized Gain On Real Estate Fund Investments	45,818		45,818
Financial Assets And Liabilities Not Measured At Fair Value By Balance Sheet Grouping [Line Items]
Mezzanine loans receivable	131,585		131,585			133,948
Notes and mortgages payable	7,852,657		7,852,657			8,065,576
Senior unsecured notes	1,357,921		1,357,921			1,357,661
Exchangeable senior debentures	0		0			497,898
Convertible senior debentures	0		0			10,168
Revolving credit facility debt	600,000		600,000			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	131,585		131,585			133,948
Total Assets	131,585		131,585			133,948
Notes and mortgages payable	7,852,657		7,852,657			8,065,576
Senior unsecured notes	1,357,921		1,357,921			1,357,661
Exchangeable senior debentures	0		0			497,898
Convertible senior debentures	0		0			10,168
Revolving credit facility debt	600,000		600,000			138,000
Total Debt	9,810,578		9,810,578			10,069,303
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	127,000		127,000			129,000
Stock purchase warrant (residential property)	35,000		35,000			0
Total Assets	162,000		162,000			129,000
Notes and mortgages payable	7,900,000		7,900,000			8,181,000
Senior unsecured notes	1,476,000		1,476,000			1,426,000
Exchangeable senior debentures	0		0			510,000
Convertible senior debentures	0		0			10,000
Revolving credit facility debt	600,000		600,000			138,000
Total Debt	9,976,000		9,976,000			10,265,000
Fair Value, Measurements, Recurring [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Marketable Securities	485,001		485,001			741,321
Derivative position in marketable equity securities	8,524		8,524			30,600
Deferred Compensation Plan Assets	103,003		103,003			95,457
Real Estate Fund investments	482,442		482,442			346,650
Total assets	1,078,970		1,078,970			1,214,028
Mandatorily redeemable instruments (included in other liabilities)	55,097		55,097			54,865
Interest rate swap (included in other liabilities)	52,935		52,935			44,114
Total liabilities	108,032		108,032			98,979
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Marketable Securities	485,001		485,001			741,321
Derivative position in marketable equity securities	0		0			0
Deferred Compensation Plan Assets	42,236		42,236			39,236
Real Estate Fund investments	0		0			0
Total assets	527,237		527,237			780,557
Mandatorily redeemable instruments (included in other liabilities)	55,097		55,097			54,865
Interest rate swap (included in other liabilities)	0		0			0
Total liabilities	55,097		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			0
Derivative position in marketable equity securities	8,524		8,524			30,600
Deferred Compensation Plan Assets	0		0			0
Real Estate Fund investments	0		0			0
Total assets	8,524		8,524			30,600
Mandatorily redeemable instruments (included in other liabilities)	0		0			0
Interest rate swap (included in other liabilities)	52,935		52,935			44,114
Total liabilities	52,935		52,935			44,114
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 position in marketable equity securities	0		0			0
Deferred Compensation Plan Assets	60,767		60,767			56,221
Real Estate Fund investments	482,442		482,442			346,650
Total assets	543,209		543,209			402,871
Mandatorily redeemable instruments (included in other liabilities)	0		0			0
Interest rate swap (included in other liabilities)	0		0			0
Total liabilities	0		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"	482,442	261,417	482,442	261,417	388,455	346,650	255,795	144,423
Fair Value Asset Measured on Recurring Basis, Asset, Purchases	88,429	0	163,021	123,047
Fair Value Asset Measured On Recurring Basis Asset Sales	0	0	(61,052)	(12,831)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Realized Gain Loss Included In Earnings	0	0	0	3,085
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Unrealized Gain Loss Included In Earnings	5,558	5,639	33,537	19,209
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Other Net	0	(17)	286	(15,516)
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 1.8 to 6.4 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			5.30%
Fair Value Inputs, Holding Period			1 year 9 months 18 days
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 4 months 24 days
Real Estate Fund [Member] | Weighted Average [Member]
Unobservable Quantitative Input [Abstract]
Fair Value Inputs, Discount Rate			14.60%
Fair Value Inputs, Cap Rate			6.00%
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"	60,767	53,887	60,767	53,887	58,313	56,221	53,724	47,850
Fair Value Asset Measured on Recurring Basis, Asset, Purchases	1,650	3,155	5,416	22,259
Fair Value Asset Measured On Recurring Basis Asset Sales	(276)	(1,044)	(4,287)	(18,538)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	1,080	(2,051)	3,349	2,166
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Other Net	$ 0	$ 103	$ 68	$ 150
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.