CONSOLIDATED STATEMENTS OF ASSETS AND LIABILITIES (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Securities loaned	$ 0.5	$ 91.2
Payable for collateral for securities loaned	0.5	93.0
Real estate properties at cost	12,715.0	12,818.1
Real estate joint ventures and limited partnerships at cost	4,803.8	4,530.4
Real estate-related marketable securities at cost	881.9	883.9
Other marketable securities at cost	4,090.6	4,054.0
Loans Receivable at cost	295.8	294.8
Cost of total investments	22,787.1	22,581.2
Principal outstanding on mortgage loans payable	$ 2,316.3	$ 2,316.5