Fair Value of Derivatives and Financial Instruments (Details) (USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Contingent consideration accrued in business combination	$ 3,000
Level 1 [Member] \| Corporate Debt Securities [Member] \| Fair Value, Measurements, Recurring [Member]
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Available-for-sale equity securities		3,866
Level 1 [Member] \| Interest Rate Contracts [Member] \| Fair Value, Measurements, Recurring [Member]
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Derivatives - Interest rate contracts	0	0
Derivatives - Interest rate contracts	0	0
Level 1 [Member] \| Mortgage notes payable [Member] \| Fair Value, Measurements, Recurring [Member]
Secured debt instruments - fixed rate
Secured debt instruments - fixed rate	0	0
Secured debt instruments - variable rate
Secured debt instruments - variable rate	0	0
Level 1 [Member] \| Tax Exempt Notes Payable [Member] \| Fair Value, Measurements, Recurring [Member]
Secured debt instruments - fixed rate
Secured debt instruments - fixed rate		0
Secured debt instruments - variable rate
Secured debt instruments - variable rate	0	0
Level 1 [Member] \| Credit Facilities [Member] \| Fair Value, Measurements, Recurring [Member]
Secured debt instruments - fixed rate
Secured debt instruments - fixed rate	0	0
Secured debt instruments - variable rate
Secured debt instruments - variable rate	0	0
Level 1 [Member] \| Commercial bank [Member] \| Fair Value, Measurements, Recurring [Member]
Unsecured debt instruments
Unsecured debt instruments	0	0
Level 1 [Member] \| Unsecured Debt [Member] \| Fair Value, Measurements, Recurring [Member]
Unsecured debt instruments
Unsecured debt instruments	0	264,849
Level 1 [Member] \| Fair Value, Measurements, Recurring [Member]
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Total assets	0	3,866
Contingent consideration accrued in business combination	0	0
Unsecured debt instruments
Total liabilities	0	264,849
Level 2 [Member] \| Corporate Debt Securities [Member] \| Fair Value, Measurements, Recurring [Member]
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Available-for-sale equity securities		0
Level 2 [Member] \| Interest Rate Contracts [Member] \| Fair Value, Measurements, Recurring [Member]
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Derivatives - Interest rate contracts	50	514
Derivatives - Interest rate contracts	15,305	6,597
Level 2 [Member] \| Mortgage notes payable [Member] \| Fair Value, Measurements, Recurring [Member]
Secured debt instruments - fixed rate
Secured debt instruments - fixed rate	0	0
Secured debt instruments - variable rate
Secured debt instruments - variable rate	0	0
Level 2 [Member] \| Tax Exempt Notes Payable [Member] \| Fair Value, Measurements, Recurring [Member]
Secured debt instruments - fixed rate
Secured debt instruments - fixed rate		0
Secured debt instruments - variable rate
Secured debt instruments - variable rate	0	0
Level 2 [Member] \| Credit Facilities [Member] \| Fair Value, Measurements, Recurring [Member]
Secured debt instruments - fixed rate
Secured debt instruments - fixed rate	0	0
Secured debt instruments - variable rate
Secured debt instruments - variable rate	0	0
Level 2 [Member] \| Commercial bank [Member] \| Fair Value, Measurements, Recurring [Member]
Unsecured debt instruments
Unsecured debt instruments	0	0
Level 2 [Member] \| Unsecured Debt [Member] \| Fair Value, Measurements, Recurring [Member]
Unsecured debt instruments
Unsecured debt instruments	0	0
Level 2 [Member] \| Fair Value, Measurements, Recurring [Member]
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Total assets	50	514
Contingent consideration accrued in business combination	0	0
Unsecured debt instruments
Total liabilities	15,305	6,597
Level 3 [Member] \| Corporate Debt Securities [Member] \| Fair Value, Measurements, Recurring [Member]
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Available-for-sale equity securities		0
Level 3 [Member] \| Interest Rate Contracts [Member] \| Fair Value, Measurements, Recurring [Member]
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Derivatives - Interest rate contracts	0	0
Derivatives - Interest rate contracts	0	0
Level 3 [Member] \| Mortgage notes payable [Member] \| Fair Value, Measurements, Recurring [Member]
Secured debt instruments - fixed rate
Secured debt instruments - fixed rate	661,031	306,515
Secured debt instruments - variable rate
Secured debt instruments - variable rate	154,870	405,641
Level 3 [Member] \| Tax Exempt Notes Payable [Member] \| Fair Value, Measurements, Recurring [Member]
Secured debt instruments - fixed rate
Secured debt instruments - fixed rate		13,885
Secured debt instruments - variable rate
Secured debt instruments - variable rate	94,700	94,700
Level 3 [Member] \| Credit Facilities [Member] \| Fair Value, Measurements, Recurring [Member]
Secured debt instruments - fixed rate
Secured debt instruments - fixed rate	949,687	927,413
Secured debt instruments - variable rate
Secured debt instruments - variable rate	260,451	260,450
Level 3 [Member] \| Commercial bank [Member] \| Fair Value, Measurements, Recurring [Member]
Unsecured debt instruments
Unsecured debt instruments	362,000	31,750
Level 3 [Member] \| Unsecured Debt [Member] \| Fair Value, Measurements, Recurring [Member]
Unsecured debt instruments
Unsecured debt instruments	1,674,563	1,360,643
Level 3 [Member] \| Fair Value, Measurements, Recurring [Member]
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Total assets	0	0
Contingent consideration accrued in business combination	8,402	5,402
Unsecured debt instruments
Total liabilities	4,165,704	3,406,399
Corporate Debt Securities [Member] \| Fair Value, Measurements, Recurring [Member]
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Available-for-sale equity securities		3,866
Interest Rate Contracts [Member] \| Fair Value, Measurements, Recurring [Member]
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Derivatives - Interest rate contracts	50	514
Derivatives - Interest rate contracts	15,305	6,597
Mortgage notes payable [Member] \| Fair Value, Measurements, Recurring [Member]
Secured debt instruments - fixed rate
Secured debt instruments - fixed rate	661,031	306,515
Secured debt instruments - variable rate
Secured debt instruments - variable rate	154,870	405,641
Tax Exempt Notes Payable [Member] \| Fair Value, Measurements, Recurring [Member]
Secured debt instruments - fixed rate
Secured debt instruments - fixed rate		13,885
Secured debt instruments - variable rate
Secured debt instruments - variable rate	94,700	94,700
Credit Facilities [Member] \| Fair Value, Measurements, Recurring [Member]
Secured debt instruments - fixed rate
Secured debt instruments - fixed rate	949,687	927,413
Secured debt instruments - variable rate
Secured debt instruments - variable rate	260,451	260,450
Commercial bank [Member] \| Fair Value, Measurements, Recurring [Member]
Unsecured debt instruments
Unsecured debt instruments	362,000	31,750
Unsecured Debt [Member] \| Fair Value, Measurements, Recurring [Member]
Unsecured debt instruments
Unsecured debt instruments	1,674,563	1,625,492
Fair Value, Measurements, Recurring [Member]
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Total assets	50	4,380
Contingent consideration accrued in business combination	8,402	5,402
Unsecured debt instruments
Total liabilities	$ 4,181,009	$ 3,677,845

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