Description of Business and Organization - Narrative (Details)	9 Months Ended
	Sep. 30, 2015 ft² parcel office_and_property
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Area of real estate property	10,100,000
OP units owned by the Company, percent	44.10%
Office Building
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Area of real estate property	9,400,000
Number of properties | office_and_property	14
Office Building | Manhattan
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Area of real estate property	7,500,000
Number of properties | office_and_property	9
Office Building | Fairfield County, Connecticut and Westchester County, New York [Member]
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Area of real estate property	1,900,000
Number of properties | office_and_property	5
Development Parcel
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Number of properties | parcel	3
Retail Site | Manhattan and Westport, Connecticut [Member]
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Area of real estate property	204,452
Retail Site | Manhattan
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Area of real estate property	519,845
Number of properties | office_and_property	4
Retail Site | Westport, Connecticut
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Number of properties | office_and_property	2
Other Property | Stamford, Connecticut [Member]
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Area of real estate property	380,000