Fair Value Measurements (Fair Value Measurements on a Nonrecurring Basis) (Details) - Real estate assets - Nonrecurring $ in Thousands	Sep. 30, 2020 USD ($)
Financial Assets and Liabilities Measured at Fair Value on a Non-Recurring Basis
Investment in Fifth Avenue and Times Square JV	$ 2,811,374
Level 1
Financial Assets and Liabilities Measured at Fair Value on a Non-Recurring Basis
Investment in Fifth Avenue and Times Square JV	0
Level 2
Financial Assets and Liabilities Measured at Fair Value on a Non-Recurring Basis
Investment in Fifth Avenue and Times Square JV	0
Level 3
Financial Assets and Liabilities Measured at Fair Value on a Non-Recurring Basis
Investment in Fifth Avenue and Times Square JV	$ 2,811,374