Fair Value of Derivatives and Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012	Jun. 30, 2012
Unsecured debt instruments
Redeemable Noncontrolling Interest, Equity, Common, Carrying Amount	$ 237,653		$ 223,418
Fair Value, Measurements, Recurring [Member]
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Notes receivable	67,618
Total assets	67,619
Unsecured debt instruments
Total liabilities	3,536,211			3,654,305
Redeemable Noncontrolling Interest, Equity, Common, Carrying Amount	237,653			223,418
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	1	[1]
Derivatives - Interest rate contracts	7,560	[1]		11,022	[1]
Mortgages Notes Payable [Member] | Fair Value, Measurements, Recurring [Member]
Secured debt instruments - fixed rate
Secured debt instruments - fixed rate	471,742	[2]		494,728	[2]
Secured debt instruments - variable rate
Secured debt instruments - variable rate				37,415	[2]
Tax Exempt Notes Payable [Member] | Fair Value, Measurements, Recurring [Member]
Secured debt instruments - variable rate
Secured debt instruments - variable rate	94,700	[2]		94,700	[2]
Line of Credit [Member] | Fair Value, Measurements, Recurring [Member]
Secured debt instruments - fixed rate
Secured debt instruments - fixed rate	658,532	[2]		689,295	[2]
Secured debt instruments - variable rate
Secured debt instruments - variable rate	211,409	[2]		211,409	[2]
Commercial bank [Member] | Fair Value, Measurements, Recurring [Member]
Unsecured debt instruments
Unsecured debt instruments	228,500	[3]		76,000	[3]
Senior Unsecured Notes [Member] | Fair Value, Measurements, Recurring [Member]
Unsecured debt instruments
Unsecured debt instruments	1,863,768	[3]		2,039,736	[3]
Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Notes receivable	0
Total assets	0
Unsecured debt instruments
Total liabilities	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
Notes receivable	0
Total assets	1
Unsecured debt instruments
Total liabilities	7,560			11,022
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	67,618
Unsecured debt instruments
Total liabilities	3,528,651			3,643,283
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Fair Value, Measurements, Recurring [Member]
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Notes receivable	66,700		64,006
Total assets	66,701		64,008
Unsecured debt instruments
Total liabilities	3,402,283		3,420,355
Redeemable Noncontrolling Interest, Equity, Common, Carrying Amount	237,653		223,418
Carrying (Reported) Amount, Fair Value Disclosure [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			2
Derivatives - Interest rate contracts	7,560		11,022
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Mortgages Notes Payable [Member] | Fair Value, Measurements, Recurring [Member]
Secured debt instruments - fixed rate
Secured debt instruments - fixed rate	450,641		455,533
Secured debt instruments - variable rate
Secured debt instruments - variable rate			37,415
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Tax Exempt Notes Payable [Member] | Fair Value, Measurements, Recurring [Member]
Secured debt instruments - variable rate
Secured debt instruments - variable rate	94,700		94,700
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Line of Credit [Member] | Fair Value, Measurements, Recurring [Member]
Secured debt instruments - fixed rate
Secured debt instruments - fixed rate	628,284		631,078
Secured debt instruments - variable rate
Secured debt instruments - variable rate	211,409		211,409
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Commercial bank [Member] | Fair Value, Measurements, Recurring [Member]
Unsecured debt instruments
Unsecured debt instruments	228,500		76,000
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Senior Unsecured Notes [Member] | Fair Value, Measurements, Recurring [Member]
Unsecured debt instruments
Unsecured debt instruments	1,781,189		1,903,198
Estimate of Fair Value, Fair Value Disclosure [Member] | Fair Value, Measurements, Recurring [Member]
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Notes receivable			64,930
Total assets			64,932
Estimate of Fair Value, Fair Value Disclosure [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			2
Estimate of Fair Value, Fair Value Disclosure [Member] | Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Notes receivable			0
Total assets			0
Unsecured debt instruments
Redeemable Noncontrolling Interest, Equity, Common, Carrying Amount	0			0
Estimate of Fair Value, Fair Value Disclosure [Member] | 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
Estimate of Fair Value, Fair Value Disclosure [Member] | Level 1 [Member] | Mortgages 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
Estimate of Fair Value, Fair Value Disclosure [Member] | Level 1 [Member] | Tax Exempt Notes Payable [Member] | Fair Value, Measurements, Recurring [Member]
Secured debt instruments - variable rate
Secured debt instruments - variable rate	0			0
Estimate of Fair Value, Fair Value Disclosure [Member] | Level 1 [Member] | Line of Credit [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
Estimate of Fair Value, Fair Value Disclosure [Member] | Level 1 [Member] | Commercial bank [Member] | Fair Value, Measurements, Recurring [Member]
Unsecured debt instruments
Unsecured debt instruments	0			0
Estimate of Fair Value, Fair Value Disclosure [Member] | Level 1 [Member] | Senior Unsecured Notes [Member] | Fair Value, Measurements, Recurring [Member]
Unsecured debt instruments
Unsecured debt instruments	0			0
Estimate of Fair Value, Fair Value Disclosure [Member] | Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Notes receivable			0
Total assets			2
Estimate of Fair Value, Fair Value Disclosure [Member] | 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	1		2
Derivatives - Interest rate contracts	7,560
Estimate of Fair Value, Fair Value Disclosure [Member] | Level 2 [Member] | Mortgages 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
Estimate of Fair Value, Fair Value Disclosure [Member] | Level 2 [Member] | Tax Exempt Notes Payable [Member] | Fair Value, Measurements, Recurring [Member]
Secured debt instruments - variable rate
Secured debt instruments - variable rate	0			0
Estimate of Fair Value, Fair Value Disclosure [Member] | Level 2 [Member] | Line of Credit [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
Estimate of Fair Value, Fair Value Disclosure [Member] | Level 2 [Member] | Commercial bank [Member] | Fair Value, Measurements, Recurring [Member]
Unsecured debt instruments
Unsecured debt instruments	0			0
Estimate of Fair Value, Fair Value Disclosure [Member] | Level 2 [Member] | Senior Unsecured Notes [Member] | Fair Value, Measurements, Recurring [Member]
Unsecured debt instruments
Unsecured debt instruments	0			0
Estimate of Fair Value, Fair Value Disclosure [Member] | Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Estimated fair values of the financial instruments either recorded or disclosed on a recurring basis
Notes receivable	67,618		64,930
Total assets			64,930
Unsecured debt instruments
Redeemable Noncontrolling Interest, Equity, Common, Carrying Amount	0			0
Estimate of Fair Value, Fair Value Disclosure [Member] | 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
Estimate of Fair Value, Fair Value Disclosure [Member] | Level 3 [Member] | Mortgages Notes Payable [Member] | Fair Value, Measurements, Recurring [Member]
Secured debt instruments - fixed rate
Secured debt instruments - fixed rate	471,742			494,728
Secured debt instruments - variable rate
Secured debt instruments - variable rate				37,415
Estimate of Fair Value, Fair Value Disclosure [Member] | Level 3 [Member] | Tax Exempt Notes Payable [Member] | Fair Value, Measurements, Recurring [Member]
Secured debt instruments - variable rate
Secured debt instruments - variable rate	94,700			94,700
Estimate of Fair Value, Fair Value Disclosure [Member] | Level 3 [Member] | Line of Credit [Member] | Fair Value, Measurements, Recurring [Member]
Secured debt instruments - fixed rate
Secured debt instruments - fixed rate	658,532			689,295
Secured debt instruments - variable rate
Secured debt instruments - variable rate	211,409			211,409
Estimate of Fair Value, Fair Value Disclosure [Member] | Level 3 [Member] | Commercial bank [Member] | Fair Value, Measurements, Recurring [Member]
Unsecured debt instruments
Unsecured debt instruments	228,500			76,000
Estimate of Fair Value, Fair Value Disclosure [Member] | Level 3 [Member] | Senior Unsecured Notes [Member] | Fair Value, Measurements, Recurring [Member]
Unsecured debt instruments
Unsecured debt instruments	$ 1,863,768			$ 2,039,736

[1] See Note 6, Secured and Unsecured Debt

[2] See Note 2, Significant Accounting Policies

[3] See Note 10, Derivatives and Hedging Activity