Organization of The Parent Company and The Operating Partnership (Textual) (Details)	Jan. 10, 2018 Real_Estate_Investment	Dec. 31, 2017	Dec. 31, 2017 property	Dec. 31, 2017 ft²	Dec. 31, 2017 Real_Estate_Investment	Dec. 31, 2017 a	Dec. 31, 2017 Apartment_unit	Dec. 31, 2016 ft²	Dec. 31, 2015 ft²
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Properties | property			93
Assets Held-for-sale [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Acreage of land | a						13.1
Subsequent Event [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Investments in Real Estate Ventures | Real_Estate_Investment	10
Office Properties [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Properties | property			81
Rentable Square Feet								5,239,632	2,649,926
Mixed Use Properties [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Properties | property			5
Retail Properties [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Properties | property			1
Core Properties [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Properties | property			87
Development Property [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Properties | property			2
Redevelopment Properties [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Properties | property			4
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				6,700,000
Unconsolidated Real Estate Ventures [Member] | Three 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					3
Acreage of land | a						4.4
Unconsolidated Real Estate Ventures [Member] | One Real Estate Venture [Member] | Active Development Land [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Number of Unconsolidated Investments in Real Estate Ventures | Real_Estate_Investment					1
Acreage of land | a						1.3
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					1		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					1		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				16,400,000
Undeveloped Parcels of Land | a						212
Undeveloped Land Held for Sale | a						13.1
Area of Additional Undeveloped Parcels of Land With Option to Purchase | a						60
Total Potential Development Capacity				14,800,000
Parent Company [Member] | Assets Held-for-sale [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Total Potential Development Capacity				100,000
Wholly-owned Management Company Subsidiaries [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Net Rentable Square Feet				25,200,000
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				8,800,000
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				16,400,000
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				8,800,000
Subsidiaries [Member]
Organization of The Parent Company and The Operating Partnership [Line Items]
Ownership in the Operating Partnership		100.00%