Organization of The Parent Company and The Operating Partnership (Textual) (Details) - Sep. 30, 2017 ft² in Millions	Total	property	ft²	Real_Estate_Investment	a	Apartment_unit
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Properties		97
Office Properties [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Properties		82
Mixed Use Properties [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Properties		4
Retail Properties [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Properties		1
Core Properties [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Properties		87
Development Property [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Properties		1
Redevelopment Properties [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Properties		3
Assets Held-for-sale [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Properties		6
Unconsolidated Real Estate Ventures [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Unconsolidated Investments in Real Estate Ventures | Real_Estate_Investment				13
Unconsolidated Real Estate Ventures [Member] | Seven Real Estate Ventures [Member] | Office Properties [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Unconsolidated Investments in Real Estate Ventures | Real_Estate_Investment				7
Rentable Square Feet | ft²			7.8
Unconsolidated Real Estate Ventures [Member] | Four Real Estate Ventures [Member] | Land Held For Development [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Unconsolidated Investments in Real Estate Ventures | Real_Estate_Investment				4
Acreage of land | a					5.7
Unconsolidated Real Estate Ventures [Member] | Two Other Real Estate Ventures [Member] | Residential Tower One [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Unconsolidated Investments in Real Estate Ventures | Real_Estate_Investment				1
Number of Property units		345				345
Unconsolidated Real Estate Ventures [Member] | Two Other Real Estate Ventures [Member] | Residential Tower Two [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Unconsolidated Investments in Real Estate Ventures | Real_Estate_Investment				1
Number of Property units		321				321
Parent Company [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Ownership in the Operating Partnership	99.20%
Net Rentable Square Feet | ft²			16.4
Area Owned by Company of Undeveloped Parcels of Land | a					214
Area of Additional Undeveloped Parcels of Land With Option to Purchase | a					60
Total Potential Development Capacity | ft²			15.7
Wholly-owned Management Company Subsidiaries [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Net Rentable Square Feet | ft²			26.4
Wholly-owned Management Company Subsidiaries [Member] | Partially Owned Properties [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Net Rentable Square Feet | ft²			10.0
Wholly-owned Management Company Subsidiaries [Member] | Wholly Owned Properties [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Net Rentable Square Feet | ft²			16.4
Wholly-owned Management Company Subsidiaries [Member] | Office and Industrial Properties [Member] | Partially Owned Properties [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Net Rentable Square Feet | ft²			10.0