Assets and Liabilities Measured at Fair Value on a Recurring Basis - Schedule of net unrealized gains included in changes in net assets attributable to investments and mortgage loans payable (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total realized and unrealized gains included in changes in net assets	$ 151.6	$ 83.4	$ 229.2	$ 202.3
Mortgage Loans Payable
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total realized and unrealized gains included in changes in net assets	(36.7)	27.5	(66.3)	55.2
Real Estate Properties
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total realized and unrealized gains included in changes in net assets	170.3	16.1	244.2	110.7
Real Estate Joint Ventures
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total realized and unrealized gains included in changes in net assets	(14.7)	67.4	(12.2)	91.6
Loans Receivable
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total realized and unrealized gains included in changes in net assets	$ (4.0)	$ (0.1)	$ (2.8)	$ 0.0