Properties Held for Sale and Property Dispositions (Details) $ in Millions	1 Months Ended
	Sep. 30, 2020 USD ($) ft²	May 31, 2020 USD ($) ft²	Mar. 31, 2020 USD ($) ft²
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Approximate Square Feet | ft²	30,328,709
315 West 33rd Street
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Approximate Square Feet | ft²			492,987
Sales Price			$ 446.5
Gain (loss)			$ 72.3
Accrued employee compensation in connection with realization of investment	$ 6.0
609 Fifth Avenue
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Approximate Square Feet | ft²		21,437
Sales Price		$ 168.0
Gain (loss)		$ 65.4
Accrued employee compensation in connection with realization of investment	$ 2.0
400 East 58th Street
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Approximate Square Feet | ft²	140,000
Sales Price	$ 62.0
Gain (loss)	$ 8.3