Mortgages and Notes Payable (Details) (USD $)							9 Months Ended		9 Months Ended
	Sep. 30, 2011	Dec. 31, 2010	Sep. 30, 2011 Secured indebtedness [Member]	Dec. 31, 2010 Secured indebtedness [Member]	Sep. 30, 2011 Unsecured indebtedness [Member]	Dec. 31, 2010 Unsecured indebtedness [Member]	Sep. 30, 2011 Unsecured revolving credit facility [Member]	Oct. 20, 2011 Unsecured revolving credit facility [Member]	Sep. 30, 2011 Secured construction facility [Member]
Debt Instrument [Line Items]
Total mortgages and notes payable	$ 1,893,981,000	$ 1,522,945,000	$ 937,846,000	$ 754,399,000	$ 956,135,000	$ 768,546,000
Aggregate undepreciated book value of secured real estate assets	1,500,000,000
Line Of Credit Facility [Line Items]
Maximum borrowing capacity							475,000,000
Maximum borrowing capacity for previous revolving credit facility							400,000,000
Maturity date							Jul. 27, 2015		Dec. 20, 2011
Additional borrowing capacity							75,000,000
Interest rate							LIBOR plus 150 basis points		LIBOR plus 85 basis points
Annual facility fee (in hundredths)							0.35%
Amount outstanding							165,000,000	347,000,000	52,100,000
Outstanding letters of credit							200,000	200,000
Unused borrowing capacity							$ 309,800,000	$ 127,800,000
Term of optional extension (in years)							1		1