Fair Value Measurements (Fair Value Measurements on a Nonrecurring Basis) (Details) - Real Estate - Nonrecurring - USD ($) $ in Thousands	Dec. 31, 2020	Sep. 30, 2020
Financial Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis
Real estate assets	$ 191,116
Fifth Avenue and Times Square JV
Financial Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis
Real estate assets		$ 2,811,374
Level 1
Financial Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis
Real estate assets	0
Level 1 | Fifth Avenue and Times Square JV
Financial Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis
Real estate assets		0
Level 2
Financial Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis
Real estate assets	0
Level 2 | Fifth Avenue and Times Square JV
Financial Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis
Real estate assets		0
Level 3
Financial Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis
Real estate assets	$ 191,116
Level 3 | Fifth Avenue and Times Square JV
Financial Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis
Real estate assets		$ 2,811,374