Description of Business and Organization - Narrative (Details) - Jun. 30, 2015	ft² parcel office_and_property
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	42.70%
Office Building [Member]
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Area of real estate property	9,300,000
Number of properties | office_and_property	14
Office Building [Member] | Manhattan [Member]
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 [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 | office_and_property	5
Development Parcel [Member]
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Number of properties | parcel	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	515,374
Number of properties | office_and_property	4
Retail Site [Member] | Westport, Connecticut [Member]
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Number of properties | 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	380,000