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, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Real estate properties		$ 15,401.5	$ 15,452.8
Marketable securities:
Real estate-related	[1]	1,097.5	1,081.5
Marketable Securities Excluding Real Estate		4,089.8	4,053.8
Total Investments		26,960.9	26,643.7
Mortgage loans payable		(2,320.3)	(2,332.1)
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Real estate properties		15,401.5	15,452.8
Real estate joint ventures		5,936.5	5,622.4
Limited partnerships		138.9	137.5
Marketable securities:
Real estate-related		1,097.5	1,081.5
Loans receivable		296.7	295.7
Total Investments		26,960.9	26,643.7
Mortgage loans payable		(2,320.3)	(2,332.1)
Fair Value, Measurements, Recurring | Government agency notes
Marketable securities:
Marketable Securities Excluding Real Estate		2,655.3	2,308.9
Fair Value, Measurements, Recurring | United States Treasury securities
Marketable securities:
Marketable Securities Excluding Real Estate		1,434.5	1,744.9
Fair Value, Measurements, Recurring | Level 1: Quoted Prices in Active Markets for Identical Assets
Marketable securities:
Real estate-related		1,097.5	1,081.5
Total Investments		1,097.5	1,081.5
Fair Value, Measurements, Recurring | Level 2: Significant Other Observable Inputs
Marketable securities:
Total Investments		4,089.8	4,053.8
Fair Value, Measurements, Recurring | Level 2: Significant Other Observable Inputs | Government agency notes
Marketable securities:
Marketable Securities Excluding Real Estate		2,655.3	2,308.9
Fair Value, Measurements, Recurring | Level 2: Significant Other Observable Inputs | United States Treasury securities
Marketable securities:
Marketable Securities Excluding Real Estate		1,434.5	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		15,401.5	15,452.8
Real estate joint ventures		5,936.5	5,622.4
Marketable securities:
Loans receivable		296.7	295.7
Total Investments		21,634.7	21,370.9
Mortgage loans payable		$ (2,320.3)	$ (2,332.1)

[1] Includes securities loaned of $0.5 million at March 31, 2017 and $91.2 million at December 31, 2016.