Fair Value of Financial Assets and Liabilities (Details) (USD $)							9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 Carrying value per balance sheet	Dec. 31, 2012 Carrying value per balance sheet	Sep. 30, 2013 Estimated fair value	Dec. 31, 2012 Estimated fair value	Sep. 30, 2013 Estimated fair value Low end of the range	Sep. 30, 2013 Estimated fair value Median of Range	Sep. 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	$ 21,100,000	$ 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			811,100,000	175,900,000	798,100,000	176,700,000
Notes payable			$ 3,200,000,000	$ 2,550,000,000	$ 3,369,100,000	$ 2,827,100,000
Length of Treasury yield curve used to calculate interest rate							5 years	7 years	10 years