CONSOLIDATED STATEMENTS OF ASSETS AND LIABILITIES (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Securities loaned	$ 6.9	$ 67.4
Cash collateral for securities loaned	7.1	68.8
Real estate properties at cost	13,384.4	12,687.8
Real estate joint ventures and limited partnerships at cost	5,588.7	5,207.8
Real estate-related Marketable securities at cost	719.7	1,274.7
Other Marketable securities at cost	4,198.7	4,088.9
Loans receivable at cost	1,105.5	910.6
Cost of Total investments	24,997.0	24,169.8
Principal outstanding on Mortgage loans payable	$ 2,788.5	$ 2,688.1