Description of Business and Organization - Narrative (Details)	12 Months Ended
	Dec. 31, 2018 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	57.70%
Number of entities to be treated as taxable REIT subsidiary | entity	2
Office
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 | Manhattan
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 | Fairfield County, Connecticut and Westchester County, New York
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Area of real estate property (in sqft)	1,800,000
Number of offices and properties (property) | office_and_property	5
Development Parcel
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Number of offices and properties (property) | parcel	3
Retail | Manhattan and Westport, Connecticut
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Area of real estate property (in sqft)	205,748
Retail | Manhattan
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Area of real estate property (in sqft)	513,606
Number of offices and properties (property) | office_and_property	4
Retail | Westport, Connecticut
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Number of offices and properties (property) | office_and_property	2
Other Property | Stamford, Connecticut
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Area of real estate property (in sqft)	380,000