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		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
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	$ 32.5	$ 89.6	$ 133.8	$ 349.7
Mortgage Loans Payable
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(2,320.3)	(1,795.8)	(2,332.1)	(1,794.4)
Total realized and unrealized gains (losses) included in changes in net assets	(18.1)	(24.1)	(6.5)	(25.7)
Purchases	0.0	(563.5)	0.0	(563.5)
Settlements	48.3	0.2	48.5	0.4
Ending balance	(2,290.1)	(2,383.2)	(2,290.1)	(2,383.2)
Real Estate Properties
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	15,401.5	14,762.2	15,452.8	14,606.2
Total realized and unrealized gains (losses) included in changes in net assets	32.8	114.8	67.4	215.4
Purchases	62.3	258.9	91.8	404.6
Sales	0.0	(4.6)	(115.4)	(94.9)
Ending balance	15,496.6	15,131.3	15,496.6	15,131.3
Real Estate Joint Ventures
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	5,936.5	4,221.0	5,622.4	4,068.4
Total realized and unrealized gains (losses) included in changes in net assets	0.1	(17.2)	62.8	133.5
Purchases	10.8	36.6	262.5	38.7
Settlements	(0.6)	(1.5)	(0.9)	(1.7)
Ending balance	5,946.8	4,238.9	5,946.8	4,238.9
Loans Receivable
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	296.7	100.6	295.7	100.6
Total realized and unrealized gains (losses) included in changes in net assets	0.0	0.0	0.0	0.0
Purchases	0.6	0.0	1.6	0.0
Settlements		0.0		0.0
Ending balance	297.3	100.6	297.3	100.6
Total Level 3 Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	21,634.7	19,083.8	21,370.9	18,775.2
Total realized and unrealized gains (losses) included in changes in net assets	32.9	97.6	130.2	348.9
Purchases	73.7	295.5	355.9	443.3
Sales	0.0	(4.6)	(115.4)	(94.9)
Settlements	(0.6)	(1.5)	(0.9)	(1.7)
Ending balance	$ 21,740.7	$ 19,470.8	$ 21,740.7	$ 19,470.8