Mortgages and Notes Payable (Details) (USD $)	3 Months Ended		6 Months Ended			3 Months Ended	6 Months Ended	3 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013	Jun. 30, 2014 Revolving Credit Facility due 2018 [Member] extensions	Jun. 30, 2014 Revolving Credit Facility due 2018 [Member]	Jun. 30, 2014 3.20% (3.363% effective rate) Notes due 2021 [Member]	Jun. 30, 2014 5.21% (3.11% effective rate) Mortgage Loan due 2014 [Member]	Jun. 30, 2014 5.94% (3.32% effective rate) Mortgage Loan due 2014 [Member]	Jun. 30, 2014 Secured indebtedness [Member]	Dec. 31, 2013 Secured indebtedness [Member]	Jun. 30, 2014 Unsecured indebtedness [Member]	Dec. 31, 2013 Unsecured indebtedness [Member]	Jul. 21, 2014 Revolving Credit Facility due 2018 [Member]
Debt Instrument [Line Items]
Mortgages and notes payable	$ 2,058,972,000		$ 2,058,972,000		$ 1,956,299,000						$ 352,857,000	$ 488,664,000	$ 1,706,115,000	$ 1,467,635,000
Aggregate undepreciated book value of secured real estate assets											652,400,000
Maximum borrowing capacity on revolving credit facility						475,000,000	475,000,000
Maturity date on revolving credit facility							Jan. 01, 2018
Additional borrowing capacity on revolving credit facility						75,000,000	75,000,000
Interest rate, basis						LIBOR plus 110 basis points
Interest rate (in hundredths)						1.10%		3.20%
Annual facility fee (in hundredths)						0.20%
Term of optional extension						6 months
Number of additional maturity extensions						2
Amount outstanding on revolving credit facility						157,000,000	157,000,000								181,000,000
Outstanding letters of credit on revolving credit facility						93,000	93,000								93,000
Unused borrowing capacity on revolving credit facility						317,907,000	317,907,000								293,907,000
Principal amount of debt								300,000,000
Debt instrument maturity date								Jun. 15, 2021	Jul. 01, 2014	Aug. 01, 2014
Original issuance discount								3,100,000
Purchase price of debt (in hundredths)								98.983%
Effective percentage rate (in hundredths)								3.363%
Deferred financing fees								2,400,000
Early repayment of debt									123,700,000	7,200,000
Current interest rate (in hundredths)									3.11%	3.32%
Gains/(losses) on debt extinguishment	$ 18,000	$ 0	$ 18,000	$ (164,000)						$ 100,000