COVID-19 Pandemic - Narrative (Details) ft² in Thousands, $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 10, 2020 ft²	Dec. 31, 2019 USD ($)
Unusual or Infrequent Item, or Both [Line Items]
Deferred leasing costs	$ 38,097	$ 38,097			$ 41,123
Reduction of rental revenues		4,122	$ 0
COVID-19
Unusual or Infrequent Item, or Both [Line Items]
Rent collected, percent	95.00%
Rent collected & deferred, percent	96.00%
Reduction of rental revenues	$ 3,100	4,122
Write-off of rent receivable	$ 6,590	10,837
Office | 731 Lexington Avenue | COVID-19
Unusual or Infrequent Item, or Both [Line Items]
Rent collected, percent	100.00%
Retail | COVID-19
Unusual or Infrequent Item, or Both [Line Items]
Rent collected, percent	87.00%
Rent collected & deferred, percent	89.00%
Residential | Alexander Apartment Tower | COVID-19
Unusual or Infrequent Item, or Both [Line Items]
Rent collected, percent	97.00%
Century 21 | COVID-19
Unusual or Infrequent Item, or Both [Line Items]
Reduction of rental revenues	$ 2,716	2,716
Write-off of rent receivable	5,919	5,919
Century 21 | Rego Park II
Unusual or Infrequent Item, or Both [Line Items]
Area of property (in sqft.) | ft²				135
Annualized revenue		6,400
Deferred leasing costs	$ 1,619	$ 1,619