Description of Business and Organization - Narrative (Details)	3 Months Ended
	Mar. 31, 2015 sqft
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Area of real estate property	10,000,000
OP units owned by the Company, percent	41.80%
Office Building [Member]
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Area of real estate property	9,300,000
Number of properties	14
Office Building [Member] | Manhattan [Member]
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Area of real estate property	7,400,000
Number of properties	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	1,900,000
Number of properties	5
Development Parcel [Member]
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Number of properties	3
Retail Site [Member] | Manhattan and Westport, Connecticut [Member]
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Area of real estate property	204,175
Retail Site [Member] | Manhattan [Member]
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Area of real estate property	522,601
Number of properties	4
Retail Site [Member] | Westport, Connecticut [Member]
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Number of properties	2
Other Property [Member] | Stamford, Connecticut [Member]
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Area of real estate property	380,000