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
	Mar. 31, 2020	Mar. 31, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total realized and unrealized gains included in changes in net assets	$ 74.0	$ 77.6
Loans Payable
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total realized and unrealized gains, still held, included in changes in net assets	45.7	(29.6)
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	147.7	73.9
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	(62.6)	2.5
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	$ (11.1)	$ 1.2