COVID-19 Pandemic - Narrative (Details) ft² in Thousands, $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 10, 2020 ft²
Unusual or Infrequent Item, or Both [Line Items]
Reduction of rental revenues		$ 4,122	$ 0	$ 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		4,122
Write-off of rent receivable		10,837
Office | 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	90.00%
Rent collected & deferred, percent	91.00%
Residential | COVID-19
Unusual or Infrequent Item, or Both [Line Items]
Rent collected, percent	98.00%
Century 21 | COVID-19
Unusual or Infrequent Item, or Both [Line Items]
Reduction of rental revenues		2,716
Write-off of rent receivable		5,919
Century 21 | Rego Park II
Unusual or Infrequent Item, or Both [Line Items]
Area of property (in square feet) | ft²					135
Annualized revenue		$ 6,400