Mortgages and Notes Payable (Details) (USD $)	3 Months Ended		6 Months Ended			3 Months Ended		3 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Jun. 30, 2013 Revolving Credit Facility due 2015 [Member]	Jul. 18, 2013 Revolving Credit Facility due 2015 [Member]	Mar. 31, 2013 Variable Rate Term Loan due 2016 [Member]	Jun. 30, 2013 Secured indebtedness [Member]	Dec. 31, 2012 Secured indebtedness [Member]	Jun. 30, 2013 Unsecured indebtedness [Member]	Dec. 31, 2012 Unsecured indebtedness [Member]
Debt Instrument [Line Items]
Mortgages and notes payable	$ 1,931,655,000		$ 1,931,655,000		$ 1,859,162,000				$ 544,510,000	$ 549,607,000	$ 1,387,145,000	$ 1,309,555,000
Aggregate undepreciated book value of secured real estate assets									964,800,000
Maximum borrowing capacity on revolving credit facility						475,000,000
Maturity date on revolving credit facility						Jul. 27, 2015
Additional borrowing capacity on revolving credit facility						75,000,000
Amount outstanding on revolving credit facility						135,400,000	113,000,000
Outstanding letters of credit on revolving credit facility						100,000	100,000
Unused borrowing capacity on revolving credit facility						339,500,000	361,900,000
Early repayment of debt								35,000,000
Debt instrument maturity date								Feb. 28, 2016
Principal amount of debt								200,000,000
Loss on extinguishment of debt	$ 0	$ (973,000)	$ (164,000)	$ (973,000)				$ (200,000)