Fair Value of Financial Instruments (Details) (USD $)	Dec. 31, 2013	Dec. 31, 2012
Fair value of financial assets and liabilities
Notes receivable issued in connection with an acquisition		$ 8,780,000
Carrying value per balance sheet
Fair value of financial assets and liabilities
Notes receivable issued in connection with property sales	19,100,000	19,300,000
Notes receivable issued in connection with an acquisition		8,800,000
Mortgages payable assumed in connection with acquisitions	783,400,000	175,900,000
Notes payable, net of unamortized original issuance discounts	3,185,500,000	2,536,000,000
Estimated fair value
Fair value of financial assets and liabilities
Notes receivable issued in connection with property sales	21,100,000	20,500,000
Notes receivable issued in connection with an acquisition		8,800,000
Mortgages payable assumed in connection with acquisitions	780,000,000	176,700,000
Notes payable, net of unamortized original issuance discounts	$ 3,340,700,000	$ 2,827,100,000