Commitments and Contingencies - Major Customers and Other Concentrations (Details) - Customer concentration risk - Rental revenue	12 Months Ended
	Dec. 31, 2018 office_and_property tenant	Dec. 31, 2017 office_and_property tenant	Dec. 31, 2016 office_and_property tenant
Concentration Risk [Line Items]
Number of tenants | tenant	5	5	5
Number of properties (property) | office_and_property	6	6	6
Empire State Building
Concentration Risk [Line Items]
Concentration risk percentage	31.90%	32.00%	32.60%
One Grand Central Place
Concentration Risk [Line Items]
Concentration risk percentage	12.80%	13.10%	12.50%
111 West 33rd Street
Concentration Risk [Line Items]
Concentration risk percentage	9.30%	8.60%	6.80%
1400 Broadway
Concentration Risk [Line Items]
Concentration risk percentage	7.10%	7.40%	7.80%
First Stamford Place
Concentration Risk [Line Items]
Concentration risk percentage	5.90%	5.40%	6.40%
250 West 57th Street
Concentration Risk [Line Items]
Concentration risk percentage	5.20%	5.20%	5.30%
Customer 1
Concentration Risk [Line Items]
Concentration risk percentage	6.00%	6.30%	6.40%
Customer 2
Concentration Risk [Line Items]
Concentration risk percentage	3.10%	3.20%	3.30%
Customer 3
Concentration Risk [Line Items]
Concentration risk percentage	2.90%	2.90%	2.90%
Customer 4
Concentration Risk [Line Items]
Concentration risk percentage	2.00%	2.10%	2.30%
Customer 5
Concentration Risk [Line Items]
Concentration risk percentage	2.00%	2.00%	2.00%