Assets and Liabilities Measured at Fair Value on a Recurring Basis - Schedule of fair value assets and liabilities measured on recurring basis (Details) - USD ($) $ in Millions		Mar. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Real estate properties		$ 17,082.1	$ 15,835.0
Marketable securities:
Real estate-related	[1]	724.7	825.7
Marketable securities excluding Real estate-related		1,347.3	4,150.2
Total investments (cost: $22,939.5 and $25,697.4)		27,376.6	29,899.0
Loans payable		(2,605.6)	(2,365.0)
Line of credit		(190.0)	(250.0)
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Real estate properties		17,082.1	15,835.0
Real estate joint ventures		6,237.4	7,204.2
Real estate funds		354.1	311.8
Marketable securities:
Real estate-related		724.7	825.7
Loans receivable		1,631.0	1,572.1
Total investments (cost: $22,939.5 and $25,697.4)		27,376.6	29,899.0
Loans payable		(2,605.6)	(2,365.0)
Line of credit		(190.0)	(250.0)
Fair Value, Measurements, Recurring | Government agency notes
Marketable securities:
Marketable securities excluding Real estate-related		62.9	259.6
Fair Value, Measurements, Recurring | United States Treasury securities
Marketable securities:
Marketable securities excluding Real estate-related		749.9	2,589.1
Fair Value, Measurements, Recurring | Corporate bonds
Marketable securities:
Marketable securities excluding Real estate-related		526.7	1,268.3
Fair Value, Measurements, Recurring | Municipal bonds
Marketable securities:
Marketable securities excluding Real estate-related		7.8	33.2
Fair Value, Measurements, Recurring | Level 1: Quoted Prices in Active Markets for Identical Assets
Marketable securities:
Real estate-related		724.7	825.7
Total investments (cost: $22,939.5 and $25,697.4)		724.7	825.7
Fair Value, Measurements, Recurring | Level 2: Significant Other Observable Inputs
Marketable securities:
Total investments (cost: $22,939.5 and $25,697.4)		1,347.3	4,150.2
Fair Value, Measurements, Recurring | Level 2: Significant Other Observable Inputs | Government agency notes
Marketable securities:
Marketable securities excluding Real estate-related		62.9	259.6
Fair Value, Measurements, Recurring | Level 2: Significant Other Observable Inputs | United States Treasury securities
Marketable securities:
Marketable securities excluding Real estate-related		749.9	2,589.1
Fair Value, Measurements, Recurring | Level 2: Significant Other Observable Inputs | Corporate bonds
Marketable securities:
Marketable securities excluding Real estate-related		526.7	1,268.3
Fair Value, Measurements, Recurring | Level 2: Significant Other Observable Inputs | Municipal bonds
Marketable securities:
Marketable securities excluding Real estate-related		7.8	33.2
Fair Value, Measurements, Recurring | Level 3: Significant Unobservable Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Real estate properties		17,082.1	15,835.0
Real estate joint ventures		6,237.4	7,204.2
Marketable securities:
Loans receivable		1,631.0	1,572.1
Total investments (cost: $22,939.5 and $25,697.4)		24,950.5	24,611.3
Loans payable		(2,605.6)	(2,365.0)
Line of credit		$ (190.0)	$ (250.0)

[1] Includes securities loaned of $2.4 million at March 31, 2020 and $25.2 million at December 31, 2019.