Indebtedness - Fair Value (Details)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Book value
Fair Value of Debt
Fair value of consolidated fixed rate mortgages and unsecured indebtedness	$ 23,200,000,000	$ 22,400,000,000
Fixed-rate mortgages and unsecured indebtedness | Fair value
Fair Value of Debt
Fair value of consolidated fixed rate mortgages and unsecured indebtedness	$ 23,231,000,000	$ 22,323,000,000
Discount Rate | Weighted average | Mortgages
Fair Value of Debt
Debt percentage- measurement input	0.0375	0.0455
Discount Rate | Weighted average | Unsecured Debt
Fair Value of Debt
Debt percentage- measurement input	0.0367	0.0450