Assets and Liabilities Measured at Fair Value on a Recurring Basis - Schedule of fair value assets and liabilities measured on recurring basis using unobservable inputs (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total realized and unrealized gains (losses) included in changes in net assets	$ 74.0	$ 77.6
Loans Payable
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(2,365.0)	(2,608.0)
Total realized and unrealized gains included in changes in net assets	45.7	(29.6)
Purchases	(289.6)	0.0
Settlements	3.3	4.7
Ending balance	2,605.6	(2,632.9)
Line of Credit
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(250.0)
Purchases	(400.0)
Settlements	460.0
Ending balance	190.0
Real Estate Properties
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	15,835.0	15,531.1
Total realized and unrealized gains (losses) included in changes in net assets	137.5	73.9
Purchases	1,457.2	366.7
Sales	(347.6)	(3.1)
Ending balance	17,082.1	15,968.6
Real Estate Joint Ventures
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	7,204.2	6,356.6
Total realized and unrealized gains (losses) included in changes in net assets	(156.4)	7.1
Purchases	20.6	57.1
Settlements	(831.0)	(0.4)
Ending balance	6,237.4	6,420.4
Loans Receivable
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	1,572.1	913.0
Total realized and unrealized gains (losses) included in changes in net assets	(12.7)	1.2
Purchases	107.7	74.4
Settlements	(36.1)	(21.3)
Ending balance	1,631.0	967.3
Total Level 3 Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	24,611.3	22,800.7
Total realized and unrealized gains (losses) included in changes in net assets	(31.6)	82.2
Purchases	1,585.5	498.2
Sales	(347.6)	(3.1)
Settlements	(867.1)	(21.7)
Ending balance	$ 24,950.5	$ 23,356.3