Fair Value Measurements (Fair Value of Financial Instruments) (Details) - Gross mortgage notes payable - Fair Value, Inputs, Level 3 - USD ($) $ in Billions	Dec. 31, 2019	Dec. 31, 2018
Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt, fair value disclosure	$ 1.3	$ 1.2
Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt, fair value disclosure	$ 1.4	$ 1.2