Fair Value of Financial Instruments (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Carrying value per balance sheet
Fair value of financial assets and liabilities
Notes receivable issued in connection with property sales	$ 18.2	$ 18.3
Mortgages payable assumed in connection with acquisitions, net	799.3	852.6
Notes and bonds payable, net of unamortized original issuance discounts	3,785.7	3,785.4
Estimated fair value
Fair value of financial assets and liabilities
Notes receivable issued in connection with property sales	20.1	20.1
Mortgages payable assumed in connection with acquisitions, net	809.8	857.9
Notes and bonds payable, net of unamortized original issuance discounts	$ 4,121.5	$ 4,092.8