Debt - Fair Value (Details) $ in Millions	Sep. 30, 2020 USD ($) item	Dec. 31, 2019 USD ($) item
Fixed rate mortgages and unsecured indebtedness | Book value
Fair Value of Debt
Fair value of consolidated fixed rate mortgages and unsecured indebtedness | $	$ 23,700	$ 23,200
Fixed rate mortgages and unsecured indebtedness | Fair value
Fair Value of Debt
Fair value of consolidated fixed rate mortgages and unsecured indebtedness | $	$ 25,018	$ 23,231
Discount Rate | Weighted average | Fixed rate mortgages
Fair Value of Debt
Debt percentage- measurement input | item	0.0233	0.0375
Discount Rate | Weighted average | Unsecured Debt
Fair Value of Debt
Debt percentage- measurement input | item	0.0253	0.0367