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, 2017	Mar. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total realized and unrealized gains included in changes in net assets	$ 101.3	$ 260.1
Mortgage Loans Payable
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(2,332.1)	(1,794.4)
Total realized and unrealized gains included in changes in net assets	11.6	(1.6)
Purchases	0.0	0.0
Settlements	0.2	0.2
Ending balance	(2,320.3)	(1,795.8)
Real Estate Properties
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	15,452.8	14,606.2
Total realized and unrealized gains included in changes in net assets	34.6	100.6
Purchases	29.5	145.7
Sales	(115.4)	(90.3)
Ending balance	15,401.5	14,762.2
Real Estate Joint Ventures
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	5,622.4	4,068.4
Total realized and unrealized gains included in changes in net assets	62.7	150.7
Purchases	251.7	2.1
Settlements	(0.3)	(0.2)
Ending balance	5,936.5	4,221.0
Loans Receivable
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	295.7	100.6
Total realized and unrealized gains included in changes in net assets	0.0	0.0
Purchases	1.0	0.0
Settlements		0.0
Ending balance	296.7	100.6
Total Level 3 Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	21,370.9	18,775.2
Total realized and unrealized gains included in changes in net assets	97.3	251.3
Purchases	282.2	147.8
Sales	(115.4)	(90.3)
Settlements	(0.3)	(0.2)
Ending balance	$ 21,634.7	$ 19,083.8