Fair Value of Financial Assets and Liabilities (Details) (USD $)							6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 Carrying value per balance sheet	Dec. 31, 2012 Carrying value per balance sheet	Jun. 30, 2013 Estimated fair value	Dec. 31, 2012 Estimated fair value	Jun. 30, 2013 Estimated fair value Low end of the range	Jun. 30, 2013 Estimated fair value Median of Range	Jun. 30, 2013 Estimated fair value High end of the range
Fair value of financial assets and liabilities
Notes receivable issued in connection with property sales			$ 19,200,000	$ 19,300,000	$ 20,300,000	$ 20,500,000
Notes receivable issued in connection with an acquisition	8,780,000	8,780,000	8,800,000	8,800,000	8,800,000	8,800,000
Mortgages payable assumed in connection with acquisitions			827,100,000	175,900,000	824,500,000	176,700,000
Notes payable			$ 2,450,000,000	$ 2,550,000,000	$ 2,595,400,000	$ 2,827,100,000
Length of Treasury yield curve used to calculate interest rate							5 years	7 years	10 years