Property Acquisitions - Properties Acquired (Details) $ in Millions	1 Months Ended
	Aug. 31, 2018 USD ($) ft²	Jul. 31, 2018 USD ($) ft²	May 31, 2018 USD ($) ft²	Sep. 30, 2018 ft²
Property Acquisitions
Approximate Square Feet | ft²				30,584,045
2 Herald Square
Property Acquisitions
Approximate Square Feet | ft²			369,000
Acquisition price | $			$ 266.0
Business combination, fair value adjustment | $			$ 8.1
1231 Third Avenue
Property Acquisitions
Approximate Square Feet | ft²		39,000
Acquisition price | $		$ 55.4
Upper East Side Residential
Property Acquisitions
Approximate Square Feet | ft²	0.2
Acquisition price | $	$ 30.2