Fair Value of Derivatives and Financial Instruments (Details) (Fair Value, Measurements, Recurring [Member], USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Total assets	$ 148	$ 4,380
Contingent purchase consideration	5,402	5,402
Unsecured debt instruments
Total liabilities	3,860,675	3,677,845
Level 1 [Member] \| Corporate Debt Securities [Member]
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Total available-for-sale securities		3,866
Level 1 [Member] \| Interest Rate Contracts [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]
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]
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]
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]
Unsecured debt instruments
Unsecured debt instruments	0	0
Level 1 [Member] \| Unsecured Debt [Member]
Unsecured debt instruments
Unsecured debt instruments	97,099	264,849
Level 1 [Member]
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Total assets	0	3,866
Contingent purchase consideration	0	0
Unsecured debt instruments
Total liabilities	97,099	264,849
Level 2 [Member] \| Corporate Debt Securities [Member]
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Total available-for-sale securities		0
Level 2 [Member] \| Interest Rate Contracts [Member]
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Derivatives - Interest rate contracts	148	514
Derivatives - Interest rate contracts	9,979	6,597
Level 2 [Member] \| Mortgage notes payable [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]
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]
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]
Unsecured debt instruments
Unsecured debt instruments	0	0
Level 2 [Member] \| Unsecured Debt [Member]
Unsecured debt instruments
Unsecured debt instruments	0	0
Level 2 [Member]
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Total assets	148	514
Contingent purchase consideration	0	0
Unsecured debt instruments
Total liabilities	9,979	6,597
Level 3 [Member] \| Corporate Debt Securities [Member]
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Total available-for-sale securities		0
Level 3 [Member] \| Interest Rate Contracts [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]
Secured debt instruments - fixed rate
Secured debt instruments - fixed rate	597,645	306,515
Secured debt instruments - variable rate
Secured debt instruments - variable rate	177,201	405,641
Level 3 [Member] \| Tax Exempt Notes Payable [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]
Secured debt instruments - fixed rate
Secured debt instruments - fixed rate	931,648	927,413
Secured debt instruments - variable rate
Secured debt instruments - variable rate	260,451	260,450
Level 3 [Member] \| Commercial bank [Member]
Unsecured debt instruments
Unsecured debt instruments	5,000	31,750
Level 3 [Member] \| Unsecured Debt [Member]
Unsecured debt instruments
Unsecured debt instruments	1,681,550	1,360,643
Level 3 [Member]
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Total assets	0	0
Contingent purchase consideration	5,402	5,402
Unsecured debt instruments
Total liabilities	3,753,597	3,406,399
Corporate Debt Securities [Member]
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Total available-for-sale securities		3,866
Interest Rate Contracts [Member]
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Derivatives - Interest rate contracts	148	514
Derivatives - Interest rate contracts	9,979	6,597
Mortgage notes payable [Member]
Secured debt instruments - fixed rate
Secured debt instruments - fixed rate	597,645	306,515
Secured debt instruments - variable rate
Secured debt instruments - variable rate	177,201	405,641
Tax Exempt Notes Payable [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]
Secured debt instruments - fixed rate
Secured debt instruments - fixed rate	931,648	927,413
Secured debt instruments - variable rate
Secured debt instruments - variable rate	260,451	260,450
Commercial bank [Member]
Unsecured debt instruments
Unsecured debt instruments	5,000	31,750
Unsecured Debt [Member]
Unsecured debt instruments
Unsecured debt instruments	$ 1,778,649	$ 1,625,492

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