Financial Instruments and Fair Value Measurements - Summary or Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Carrying value
Fair value of financial assets and liabilities
Mortgages payable assumed in connection with acquisitions	$ 334.7	$ 408.4
Notes and bonds payable	7,007.0	6,317.6
Estimated fair value
Fair value of financial assets and liabilities
Mortgages payable assumed in connection with acquisitions	343.5	417.7
Notes and bonds payable	7,901.3	6,826.1
Mortgages Payable
Fair value of financial assets and liabilities
Unamortized balance of non-cash net premiums	1.9	3.0
Remaining balance of deferred financing costs at period end	1.1	1.3
Notes and bonds payable
Fair value of financial assets and liabilities
Unamortized balance of non-cash net premiums	28.2	6.3
Remaining balance of deferred financing costs at period end	$ 40.3	$ 35.9