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		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
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	$ 984.2	$ (107.9)	$ 1,967.5	$ (681.2)
Loans Payable
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(2,388.2)	(2,457.0)	(2,411.4)	(2,365.0)
Total realized and unrealized losses included in changes in net assets	(3.0)	(42.4)	11.0	(6.1)
Purchases	(39.7)	0.0	(39.7)	(289.6)
Settlements	87.7	3.7	96.9	165.0
Ending balance	(2,343.2)	(2,495.7)	(2,343.2)	(2,495.7)
Line of Credit
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance		(291.0)		(250.0)
Purchases		0.0		(540.0)
Settlements		200.0		699.0
Ending balance		(91.0)		(91.0)
Real Estate Properties
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	17,029.8	16,748.6	16,476.7	15,835.0
Total realized and unrealized gains (losses) included in changes in net assets	530.9	(92.1)	1,425.7	(334.0)
Purchases	112.8	55.4	332.6	1,564.3
Sales	(274.8)	3.1	(836.3)	(350.3)
Ending balance	17,398.7	16,715.0	17,398.7	16,715.0
Real Estate Joint Ventures
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	6,482.6	6,024.6	6,128.9	7,204.2
Total realized and unrealized gains (losses) included in changes in net assets	425.1	(45.6)	579.2	(422.1)
Purchases	180.5	15.0	380.4	57.0
Settlements	(35.1)	(0.2)	(35.4)	(845.3)
Ending balance	7,053.1	5,993.8	7,053.1	5,993.8
Real Estate Operating Business
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	273.5		250.0
Total realized and unrealized gains (losses) included in changes in net assets	12.5		34.6
Purchases	0.0		1.4
Ending balance	286.0		286.0
Loans Receivable
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	1,391.8	1,578.2	1,562.6	1,572.1
Total realized and unrealized gains (losses) included in changes in net assets	1.5	(8.8)	6.1	(32.6)
Purchases	92.7	3.5	256.4	114.8
Sales	0.0	0.0	(294.5)	(64.7)
Settlements	(37.9)	(3.3)	(82.5)	(20.0)
Ending balance	1,448.1	1,569.6	1,448.1	1,569.6
Total Level 3 Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	25,177.7	24,351.4	24,418.2	24,611.3
Total realized and unrealized gains (losses) included in changes in net assets	970.0	(146.5)	2,045.6	(788.7)
Purchases	386.0	73.9	970.8	1,736.1
Sales	(274.8)	3.1	(1,130.8)	(415.0)
Settlements	(73.0)	(3.5)	(117.9)	(865.3)
Ending balance	$ 26,185.9	$ 24,278.4	$ 26,185.9	$ 24,278.4