Assets and Liabilities Measured at Fair Value on a Recurring Basis - Schedule of Major Categories of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Real estate properties	$ 16,175.0	$ 15,975.4
Real estate joint ventures	5,318.0	5,098.8
Real estate operating business	1,842.5	1,072.6
Marketable securities:
Marketable securities excluding real estate-related	1,683.0	1,728.7
Loans receivable	318.5	690.4
Loans payable	(591.7)	(830.3)
Line of credit	(302.0)	(160.0)
Other unsecured debt
Marketable securities:
Other unsecured debt	(1,662.0)	(1,568.7)
U.S. government agency notes
Marketable securities:
Marketable securities excluding real estate-related	516.9	621.1
U.S treasury securities
Marketable securities:
Marketable securities excluding real estate-related	596.6	899.4
Reverse repurchase agreements
Marketable securities:
Marketable securities excluding real estate-related	569.5	208.2
Level 1: Quoted Prices in Active Markets for Identical Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Real estate properties	0.0	0.0
Real estate joint ventures	0.0	0.0
Real estate operating business	0.0	0.0
Marketable securities:
Loans receivable	0.0	0.0
Loans payable	0.0	0.0
Line of credit	0.0	0.0
Level 1: Quoted Prices in Active Markets for Identical Assets \| Other unsecured debt
Marketable securities:
Other unsecured debt	0.0	0.0
Level 1: Quoted Prices in Active Markets for Identical Assets \| U.S. government agency notes
Marketable securities:
Marketable securities excluding real estate-related	0.0	0.0
Level 1: Quoted Prices in Active Markets for Identical Assets \| U.S treasury securities
Marketable securities:
Marketable securities excluding real estate-related	0.0	0.0
Level 1: Quoted Prices in Active Markets for Identical Assets \| Reverse repurchase agreements
Marketable securities:
Marketable securities excluding real estate-related	0.0	0.0
Level 2: Significant Other Observable Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Real estate properties	0.0	0.0
Real estate joint ventures	0.0	0.0
Real estate operating business	0.0	0.0
Marketable securities:
Loans receivable	0.0	0.0
Loans payable	0.0	0.0
Line of credit	0.0	0.0
Level 2: Significant Other Observable Inputs \| Other unsecured debt
Marketable securities:
Other unsecured debt	(1,662.0)	(1,568.7)
Level 2: Significant Other Observable Inputs \| U.S. government agency notes
Marketable securities:
Marketable securities excluding real estate-related	516.9	621.1
Level 2: Significant Other Observable Inputs \| U.S treasury securities
Marketable securities:
Marketable securities excluding real estate-related	596.6	899.4
Level 2: Significant Other Observable Inputs \| Reverse repurchase agreements
Marketable securities:
Marketable securities excluding real estate-related	569.5	208.2
Level 3: Significant Unobservable Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Real estate properties	16,175.0	15,975.4
Real estate joint ventures	5,318.0	5,098.8
Real estate operating business	1,842.5	1,072.6
Marketable securities:
Loans receivable	318.5	690.4
Loans payable	(591.7)	(830.3)
Line of credit	(302.0)	(160.0)
Level 3: Significant Unobservable Inputs \| Other unsecured debt
Marketable securities:
Other unsecured debt	0.0	0.0
Level 3: Significant Unobservable Inputs \| U.S. government agency notes
Marketable securities:
Marketable securities excluding real estate-related	0.0	0.0
Level 3: Significant Unobservable Inputs \| U.S treasury securities
Marketable securities:
Marketable securities excluding real estate-related	0.0	0.0
Level 3: Significant Unobservable Inputs \| Reverse repurchase agreements
Marketable securities:
Marketable securities excluding real estate-related	$ 0.0	$ 0.0

Assets and Liabilities Measured at Fair Value on a Recurring Basis - Schedule of Major Categories of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($)
$ in Millions