CONSOLIDATED STATEMENTS OF ASSETS AND LIABILITIES (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Statement of Financial Position [Abstract]
Real estate properties at cost	$ 13,676.2	$ 13,986.3
Real estate joint ventures at cost	5,415.1	5,021.9
Real estate funds at cost	579.9	373.3
Real estate operating business at cost	251.6	250.2
Marketable Securities at cost	1,761.4	739.3
Loans receivable at cost	1,392.9	1,527.6
Loans receivable with related parties at cost	69.3	69.3
Total investments at cost	23,146.4	21,967.9
Principal outstanding on loans payable	$ 2,324.2	$ 2,381.3