Commitments and Contingencies - Major Customers and Other Concentrations (Details) - Customer concentration risk - Rental revenue - office_and_property	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Concentration Risk [Line Items]
Number of properties	6	6	6
Empire State Building
Concentration Risk [Line Items]
Concentration risk percentage	32.80%	32.90%	31.90%
One Grand Central Place
Concentration Risk [Line Items]
Concentration risk percentage	12.40%	12.30%	12.80%
111 West 33rd Street
Concentration Risk [Line Items]
Concentration risk percentage	10.50%	9.90%	9.30%
1400 Broadway
Concentration Risk [Line Items]
Concentration risk percentage	8.00%	7.10%	7.10%
250 West 57th Street
Concentration Risk [Line Items]
Concentration risk percentage	5.70%	5.50%	5.20%
First Stamford Place
Concentration Risk [Line Items]
Concentration risk percentage	5.40%	5.40%	5.90%
Tenant 1
Concentration Risk [Line Items]
Concentration risk percentage	6.90%	6.80%	6.00%
Tenant 2
Concentration Risk [Line Items]
Concentration risk percentage	3.50%	3.20%	3.10%
Tenant 3
Concentration Risk [Line Items]
Concentration risk percentage		3.20%	2.90%
Tenant 4
Concentration Risk [Line Items]
Concentration risk percentage			2.00%
Tenant 5
Concentration Risk [Line Items]
Concentration risk percentage			2.00%