Commitments and Contingencies - Major Customers and Other Concentrations (Details) - Customer concentration risk [Member] - Rental revenue [Member]	12 Months Ended
	Dec. 31, 2017 office_and_property tenant	Dec. 31, 2016 office_and_property tenant	Dec. 31, 2015 office_and_property tenant
Concentration Risk [Line Items]
Number of tenants | tenant	5	5	3
Number of properties (property) | office_and_property	6	6	6
Empire State Building Co. [Member]
Concentration Risk [Line Items]
Concentration risk percentage	32.00%	32.60%	31.40%
One Grand Central Place [Member]
Concentration Risk [Line Items]
Concentration risk percentage	13.10%	12.50%	12.20%
111 West 33rd Street [Member]
Concentration Risk [Line Items]
Concentration risk percentage	8.60%	6.80%	8.30%
1400 Broadway [Member]
Concentration Risk [Line Items]
Concentration risk percentage	7.40%	7.80%	7.70%
First Stamford Place [Member]
Concentration Risk [Line Items]
Concentration risk percentage	5.40%	6.40%	6.50%
250 West 57th Street [Member]
Concentration Risk [Line Items]
Concentration risk percentage	5.20%	5.30%	4.80%
Customer 1 [Member]
Concentration Risk [Line Items]
Concentration risk percentage	6.30%	6.40%	6.70%
Customer 2 [Member]
Concentration Risk [Line Items]
Concentration risk percentage	3.20%	3.30%	3.50%
Customer 3 [Member]
Concentration Risk [Line Items]
Concentration risk percentage	2.90%	2.90%	2.00%
Customer 4 [Member]
Concentration Risk [Line Items]
Concentration risk percentage	2.10%	2.30%
Customer 5 [Member]
Concentration Risk [Line Items]
Concentration risk percentage	2.00%	2.00%