Fair Value of Financial Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Carrying value per balance sheet
Fair value of financial assets and liabilities
Notes receivable issued in connection with property sales	$ 18.5	$ 19.1
Mortgages payable assumed in connection with acquisitions	916.5	783.4
Notes payable, net of unamortized original issuance discounts	3,536.0	3,185.5
Estimated fair value
Fair value of financial assets and liabilities
Notes receivable issued in connection with property sales	20.4	21.1
Mortgages payable assumed in connection with acquisitions	915.1	780.0
Notes payable, net of unamortized original issuance discounts	$ 3,828.3	$ 3,340.7