Mortgages and Notes Payable (Details) (USD $)			3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014 Revolving Credit Facility due 2015 [Member] extensions	Apr. 21, 2014 Revolving Credit Facility due 2015 [Member]	Mar. 31, 2014 Secured indebtedness [Member]	Dec. 31, 2013 Secured indebtedness [Member]	Mar. 31, 2014 Unsecured indebtedness [Member]	Dec. 31, 2013 Unsecured indebtedness [Member]
Debt Instrument [Line Items]
Mortgages and notes payable	$ 2,012,554,000	$ 1,956,299,000			$ 485,525,000	$ 488,664,000	$ 1,527,029,000	$ 1,467,635,000
Aggregate undepreciated book value of secured real estate assets					817,900,000
Maximum borrowing capacity on revolving credit facility			475,000,000
Maturity date on revolving credit facility			Jan. 01, 2018
Additional borrowing capacity on revolving credit facility			75,000,000
Number of additional maturity extensions			2
Term of optional extensions (in years)			6 months
Interest rate, basis			LIBOR plus 110 basis points
Interest rate, basis spread			1.10%
Annual facility fee (in hundredths)			0.20%
Amount outstanding on revolving credit facility			275,000,000	409,000,000
Outstanding letters of credit on revolving credit facility			100,000	100,000
Unused borrowing capacity on revolving credit facility			$ 199,900,000	$ 65,900,000