Description of Business and Organization - Narrative (Details)	12 Months Ended
	Dec. 31, 2017 ft² entity parcel office_and_property
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Area of real estate property (in sqft)	10,100,000
OP units owned by the Company, percent	53.80%
Number of entities to be treated as taxable REIT subsidiary | entity	2
Office Building [Member]
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Area of real estate property (in sqft)	9,400,000
Number of offices and properties (property) | office_and_property	14
Office Building [Member] | Manhattan [Member]
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Area of real estate property (in sqft)	7,600,000
Number of offices and properties (property) | office_and_property	9
Office Building [Member] | Fairfield County, Connecticut and Westchester County, New York [Member]
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Area of real estate property (in sqft)	1,900,000
Number of offices and properties (property) | office_and_property	5
Development Parcel [Member]
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Number of offices and properties (property) | parcel	3
Retail [Member] | Manhattan and Westport, Connecticut [Member]
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Area of real estate property (in sqft)	205,740
Retail [Member] | Manhattan [Member]
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Area of real estate property (in sqft)	507,395
Number of offices and properties (property) | office_and_property	4
Retail [Member] | Westport, Connecticut [Member]
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Number of offices and properties (property) | office_and_property	2
Other Property [Member] | Stamford, Connecticut [Member]
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Area of real estate property (in sqft)	380,000