Description of Business and Organization - Narrative (Details)	9 Months Ended
	Sep. 30, 2016 ft² 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	51.80%
Office Building
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Area of real estate property (in sqft)	9,400,000
Number of properties (in property) | office_and_property	14
Office Building | Manhattan
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Area of real estate property (in sqft)	7,600,000
Number of properties (in property) | office_and_property	9
Office Building | 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)	1,900,000
Number of properties (in property) | office_and_property	5
Development Parcel
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Number of properties (in property) | parcel	3
Retail Site | Manhattan and Westport, Connecticut
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Area of real estate property (in sqft)	204,452
Retail Site | Manhattan
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Area of real estate property (in sqft)	502,429
Number of properties (in property) | office_and_property	4
Retail Site | Westport, Connecticut
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Number of properties (in 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