Mortgages and Notes Payable (Details) (USD $)							6 Months Ended			6 Months Ended
	Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2011 Secured indebtedness [Member]	Dec. 31, 2010 Secured indebtedness [Member]	Jun. 30, 2011 Unsecured indebtedness [Member]	Dec. 31, 2010 Unsecured indebtedness [Member]	Jun. 30, 2011 Bank term loan [Member]	Jun. 30, 2011 Unsecured revolving credit facility [Member]	Jul. 20, 2011 Unsecured revolving credit facility [Member]	Jun. 30, 2011 Secured construction facility [Member]
Debt Instrument [Line Items]
Total mortgages and notes payable	$ 1,615,068,000	$ 1,522,945,000	$ 748,563,000	$ 754,399,000	$ 866,505,000	$ 768,546,000
Aggregate undepreciated book value of secured real estate assets	1,200,000,000
Term of debt instrument (in years)							3
Early repayment of debt							10,000,000
Line Of Credit Facility [Line Items]
Maximum borrowing capacity								400,000,000		52,100,000
Maturity date								Feb. 21, 2013		Dec. 20, 2011
Additional borrowing capacity								50,000,000
Interest rate								LIBOR plus 290 basis points		LIBOR plus 85 basis points
Annual facility fee (in hundredths)								0.60%
Amount outstanding								75,400,000	77,000,000	52,100,000
Outstanding letters of credit								200,000	200,000
Unused borrowing capacity								$ 324,400,000	$ 322,800,000
Term of optional extension (in years)										1