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		Jun. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Real estate properties		$ 15,496.6	$ 15,452.8
Marketable securities:
Real estate-related	[1]	1,114.7	1,081.5
Marketable Securities Excluding Real Estate		4,106.5	4,053.8
Total investments (cost: $22,898.0 and $22,581.2)		27,101.4	26,643.7
Mortgage loans payable		(2,290.1)	(2,332.1)
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Real estate properties		15,496.6	15,452.8
Real estate joint ventures		5,946.8	5,622.4
Limited partnerships		139.5	137.5
Marketable securities:
Real estate-related		1,114.7	1,081.5
Loans receivable		297.3	295.7
Total investments (cost: $22,898.0 and $22,581.2)		27,101.4	26,643.7
Mortgage loans payable		(2,290.1)	(2,332.1)
Fair Value, Measurements, Recurring | Government agency notes
Marketable securities:
Marketable Securities Excluding Real Estate		3,152.2	2,308.9
Fair Value, Measurements, Recurring | United States Treasury securities
Marketable securities:
Marketable Securities Excluding Real Estate		954.3	1,744.9
Fair Value, Measurements, Recurring | Level 1: Quoted Prices in Active Markets for Identical Assets
Marketable securities:
Real estate-related		1,114.7	1,081.5
Total investments (cost: $22,898.0 and $22,581.2)		1,114.7	1,081.5
Fair Value, Measurements, Recurring | Level 2: Significant Other Observable Inputs
Marketable securities:
Total investments (cost: $22,898.0 and $22,581.2)		4,106.5	4,053.8
Fair Value, Measurements, Recurring | Level 2: Significant Other Observable Inputs | Government agency notes
Marketable securities:
Marketable Securities Excluding Real Estate		3,152.2	2,308.9
Fair Value, Measurements, Recurring | Level 2: Significant Other Observable Inputs | United States Treasury securities
Marketable securities:
Marketable Securities Excluding Real Estate		954.3	1,744.9
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		15,496.6	15,452.8
Real estate joint ventures		5,946.8	5,622.4
Marketable securities:
Loans receivable		297.3	295.7
Total investments (cost: $22,898.0 and $22,581.2)		21,740.7	21,370.9
Mortgage loans payable		$ (2,290.1)	$ (2,332.1)

[1] Includes securities loaned of $10.2 million at June 30, 2017 and $91.2 million at December 31, 2016.