Mortgages and Notes Payable (Details) (USD $)			3 Months Ended		3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2012 Revolving Credit Facility due 2015 [Member]	Apr. 26, 2012 Revolving Credit Facility due 2015 [Member]	Mar. 31, 2012 Variable Rate Term Loan due 2016 [Member]	Mar. 31, 2012 Secured indebtedness [Member]	Dec. 31, 2011 Secured indebtedness [Member]	Mar. 31, 2012 Unsecured indebtedness [Member]	Dec. 31, 2011 Unsecured indebtedness [Member]
Debt Instrument [Line Items]
Mortgages and notes payable	$ 1,903,978,000	$ 1,903,213,000				$ 746,784,000	$ 750,049,000	$ 1,157,194,000	$ 1,153,164,000
Aggregate undepreciated book value of secured real estate assets						1,200,000,000
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
Interest rate			LIBOR plus 150 basis points
Annual facility fee (in hundredths)			0.35%
Amount outstanding on revolving credit facility			141,000,000	112,000,000
Outstanding letters of credit on revolving credit facility			200,000	200,000
Unused borrowing capacity on revolving credit facility			333,800,000	362,800,000
Principal amount of debt					$ 225,000,000
Term of debt instrument (in years)					7
Interest rate basis					LIBOR plus 190 basis points