Real Estate (Details) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Jun. 30, 2019 USD ($)	Sep. 30, 2019 USD ($) a ft² bldg	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) a ft² bldg	Sep. 30, 2018 USD ($)
Property, Plant and Equipment [Line Items]
Loss on debt extinguishment				$ (7,618)	$ 0
Net proceeds from disposition of properties/land				412,474	574,542
Repayments of Secured Debt				98,467	31,954
Acquisition of noncontrolling interest				18,513
Discontinued operations
Property, Plant and Equipment [Line Items]
Loss on debt extinguishment		$ 0	$ 0	$ (7,618)	$ 0
2019 acquisitions [Member] | Building and Building Improvements
Property, Plant and Equipment [Line Items]
Number of buildings (in buildings) | bldg		7		7
Leaseable Square Feet (in sqft) | ft²		1,490,950		1,490,950
2019 acquisitions [Member] | Acres of Developable Land
Property, Plant and Equipment [Line Items]
Acres of Developable Land (in acres) | a		203.2		203.2
2019 acquisitions [Member] | Land, Buildings and Improvements
Property, Plant and Equipment [Line Items]
Purchase Price (in thousands)				$ 230,318
2019 acquisitions [Member] | Dallas | Building and Building Improvements
Property, Plant and Equipment [Line Items]
Number of buildings (in buildings) | bldg		2		2
Leaseable Square Feet (in sqft) | ft²		509,733		509,733
2019 acquisitions [Member] | Dallas | Land, Buildings and Improvements
Property, Plant and Equipment [Line Items]
Purchase Price (in thousands)				$ 44,400
2019 acquisitions [Member] | Chicago/Minneapolis | Acres of Developable Land
Property, Plant and Equipment [Line Items]
Acres of Developable Land (in acres) | a		23.5		23.5
2019 acquisitions [Member] | Chicago/Minneapolis | Land, Buildings and Improvements
Property, Plant and Equipment [Line Items]
Purchase Price (in thousands)				$ 13,200
2019 acquisitions [Member] | Southern California | Building and Building Improvements
Property, Plant and Equipment [Line Items]
Number of buildings (in buildings) | bldg		2		2
Leaseable Square Feet (in sqft) | ft²		492,963		492,963
2019 acquisitions [Member] | Southern California | Land, Buildings and Improvements
Property, Plant and Equipment [Line Items]
Purchase Price (in thousands)				$ 86,785
2019 acquisitions [Member] | United Kingdom | Acres of Developable Land
Property, Plant and Equipment [Line Items]
Acres of Developable Land (in acres) | a		25.0		25.0
2019 acquisitions [Member] | United Kingdom | Land, Buildings and Improvements
Property, Plant and Equipment [Line Items]
Purchase Price (in thousands)				$ 2,658
2019 acquisitions [Member] | Atlanta | Acres of Developable Land
Property, Plant and Equipment [Line Items]
Acres of Developable Land (in acres) | a		154.7		154.7
2019 acquisitions [Member] | Atlanta | Land, Buildings and Improvements
Property, Plant and Equipment [Line Items]
Purchase Price (in thousands)				$ 5,000
2019 acquisitions [Member] | New Jersey | Building and Building Improvements
Property, Plant and Equipment [Line Items]
Number of buildings (in buildings) | bldg		3		3
Leaseable Square Feet (in sqft) | ft²		488,254		488,254
2019 acquisitions [Member] | New Jersey | Land, Buildings and Improvements
Property, Plant and Equipment [Line Items]
Purchase Price (in thousands)				$ 78,275
2019 acquisitions current quarter [Member] | Building and Building Improvements
Property, Plant and Equipment [Line Items]
Number of buildings (in buildings) | bldg		1		1
Leaseable Square Feet (in sqft) | ft²		203,280		203,280
2019 acquisitions current quarter [Member] | Acres of Developable Land
Property, Plant and Equipment [Line Items]
Acres of Developable Land (in acres) | a		23.5		23.5
2019 acquisitions current quarter [Member] | Land, Buildings and Improvements
Property, Plant and Equipment [Line Items]
Purchase Price (in thousands)		$ 68,700
2019 acquisitions current quarter [Member] | Chicago/Minneapolis | Acres of Developable Land
Property, Plant and Equipment [Line Items]
Acres of Developable Land (in acres) | a		23.5		23.5
2019 acquisitions current quarter [Member] | Chicago/Minneapolis | Land, Buildings and Improvements
Property, Plant and Equipment [Line Items]
Purchase Price (in thousands)		$ 13,200
2019 acquisitions current quarter [Member] | Southern California | Building and Building Improvements
Property, Plant and Equipment [Line Items]
Number of buildings (in buildings) | bldg		1		1
Leaseable Square Feet (in sqft) | ft²		203,280		203,280
2019 acquisitions current quarter [Member] | Southern California | Land, Buildings and Improvements
Property, Plant and Equipment [Line Items]
Purchase Price (in thousands)		$ 55,500
2019 acquisitions of noncontrolling interest
Property, Plant and Equipment [Line Items]
Number of Consolidated Joint Ventures		3		3
2019 acquisitions of noncontrolling interest | Land, Buildings and Improvements
Property, Plant and Equipment [Line Items]
Purchase Price (in thousands)				$ 18,500
2019 acquisitions of noncontrolling interest | Dallas | Building and Building Improvements
Property, Plant and Equipment [Line Items]
Number of buildings (in buildings) | bldg		1		1
2019 acquisitions of noncontrolling interest | Philadelphia | Building and Building Improvements
Property, Plant and Equipment [Line Items]
Number of buildings (in buildings) | bldg		4		4
2019 acquisitions of noncontrolling interest | Southern California | Building and Building Improvements
Property, Plant and Equipment [Line Items]
Number of buildings (in buildings) | bldg		1		1
2019 acquisitions of noncontrolling interest | Arizona | Acres of Developable Land
Property, Plant and Equipment [Line Items]
Acres of Developable Land (in acres) | a		21.7		21.7
2019 acquisitions of noncontrolling interest | New Jersey | Building and Building Improvements
Property, Plant and Equipment [Line Items]
Number of buildings (in buildings) | bldg		1		1
2019 sales current quarter | Disposal Group, Not Discontinued Operations | Building and Building Improvements
Property, Plant and Equipment [Line Items]
Number of buildings (in buildings) | bldg		6		6
Leaseable Square Feet (in sqft) | ft²		763,324		763,324
2019 sales current quarter | Disposal Group, Not Discontinued Operations | Acres of Developable Land
Property, Plant and Equipment [Line Items]
Acres of Developable Land (in acres) | a		75.2		75.2
2019 sales current quarter | Disposal Group, Not Discontinued Operations | Land, Buildings and Improvements
Property, Plant and Equipment [Line Items]
Gross Proceeds (in thousands)		$ 203,020
2019 sales current quarter | Disposal Group, Not Discontinued Operations | Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Gain on sale of nondepreciable assets		$ 7,000
2019 sales current quarter | Disposal Group, Not Discontinued Operations | Houston | Acres of Developable Land
Property, Plant and Equipment [Line Items]
Acres of Developable Land (in acres) | a		7.6		7.6
2019 sales current quarter | Disposal Group, Not Discontinued Operations | Houston | Land, Buildings and Improvements
Property, Plant and Equipment [Line Items]
Gross Proceeds (in thousands)		$ 1,700
2019 sales current quarter | Disposal Group, Not Discontinued Operations | Philadelphia | Building and Building Improvements
Property, Plant and Equipment [Line Items]
Number of buildings (in buildings) | bldg		2		2
Leaseable Square Feet (in sqft) | ft²		156,411		156,411
2019 sales current quarter | Disposal Group, Not Discontinued Operations | Philadelphia | Land, Buildings and Improvements
Property, Plant and Equipment [Line Items]
Gross Proceeds (in thousands)		$ 60,650
2019 sales current quarter | Disposal Group, Not Discontinued Operations | Arizona | Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Gross Proceeds (in thousands)		$ 5,500
2019 sales current quarter | Disposal Group, Not Discontinued Operations | Southeastern PA | Building and Building Improvements
Property, Plant and Equipment [Line Items]
Number of buildings (in buildings) | bldg		2		2
Leaseable Square Feet (in sqft) | ft²		180,151		180,151
2019 sales current quarter | Disposal Group, Not Discontinued Operations | Southeastern PA | Land, Buildings and Improvements
Property, Plant and Equipment [Line Items]
Gross Proceeds (in thousands)		$ 30,000
2019 sales current quarter | Disposal Group, Not Discontinued Operations | New Jersey | Acres of Developable Land
Property, Plant and Equipment [Line Items]
Acres of Developable Land (in acres) | a		67.6		67.6
2019 sales current quarter | Disposal Group, Not Discontinued Operations | New Jersey | Land, Buildings and Improvements
Property, Plant and Equipment [Line Items]
Gross Proceeds (in thousands)		$ 4,070
2019 sales current quarter | Disposal Group, Not Discontinued Operations | DC Metro | Building and Building Improvements
Property, Plant and Equipment [Line Items]
Number of buildings (in buildings) | bldg		1		1
Leaseable Square Feet (in sqft) | ft²		290,762		290,762
2019 sales current quarter | Disposal Group, Not Discontinued Operations | DC Metro | Land, Buildings and Improvements
Property, Plant and Equipment [Line Items]
Gross Proceeds (in thousands)		$ 92,500
2019 sales current quarter | Operating Segments | Disposal Group, Not Discontinued Operations | Carolinas/Richmond | Building and Building Improvements
Property, Plant and Equipment [Line Items]
Number of buildings (in buildings) | bldg		1		1
Leaseable Square Feet (in sqft) | ft²		136,000		136,000
2019 sales current quarter | Operating Segments | Disposal Group, Not Discontinued Operations | Carolinas/Richmond | Land, Buildings and Improvements
Property, Plant and Equipment [Line Items]
Gross Proceeds (in thousands)		$ 14,100
2019 sales | Philadelphia
Property, Plant and Equipment [Line Items]
Repayments of Secured Debt	$ (35,900)
2019 sales | Disposal Group, Not Discontinued Operations | Building and Building Improvements
Property, Plant and Equipment [Line Items]
Number of buildings (in buildings) | bldg		14		14
Leaseable Square Feet (in sqft) | ft²		1,287,969		1,287,969
2019 sales | Disposal Group, Not Discontinued Operations | Acres of Developable Land
Property, Plant and Equipment [Line Items]
Acres of Developable Land (in acres) | a		118.1		118.1
2019 sales | Disposal Group, Not Discontinued Operations | Land, Buildings and Improvements
Property, Plant and Equipment [Line Items]
Gross Proceeds (in thousands)				$ 417,490
2019 sales | Disposal Group, Not Discontinued Operations | Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Gain on sale of nondepreciable assets				$ 12,900
2019 sales | Disposal Group, Not Discontinued Operations | Chicago/Minneapolis | Acres of Developable Land
Property, Plant and Equipment [Line Items]
Acres of Developable Land (in acres) | a		11.3		11.3
2019 sales | Disposal Group, Not Discontinued Operations | Chicago/Minneapolis | Land, Buildings and Improvements
Property, Plant and Equipment [Line Items]
Gross Proceeds (in thousands)				$ 2,865
2019 sales | Disposal Group, Not Discontinued Operations | Florida | Building and Building Improvements
Property, Plant and Equipment [Line Items]
Number of buildings (in buildings) | bldg		3		3
Leaseable Square Feet (in sqft) | ft²		150,751		150,751
2019 sales | Disposal Group, Not Discontinued Operations | Florida | Land, Buildings and Improvements
Property, Plant and Equipment [Line Items]
Gross Proceeds (in thousands)				$ 23,400
2019 sales | Disposal Group, Not Discontinued Operations | Houston | Acres of Developable Land
Property, Plant and Equipment [Line Items]
Acres of Developable Land (in acres) | a		11.6		11.6
2019 sales | Disposal Group, Not Discontinued Operations | Houston | Land, Buildings and Improvements
Property, Plant and Equipment [Line Items]
Gross Proceeds (in thousands)				$ 3,205
2019 sales | Disposal Group, Not Discontinued Operations | Philadelphia | Building and Building Improvements
Property, Plant and Equipment [Line Items]
Number of buildings (in buildings) | bldg		3		3
Leaseable Square Feet (in sqft) | ft²		309,653		309,653
2019 sales | Disposal Group, Not Discontinued Operations | Philadelphia | Land, Buildings and Improvements
Property, Plant and Equipment [Line Items]
Gross Proceeds (in thousands)				$ 159,900
2019 sales | Disposal Group, Not Discontinued Operations | Philadelphia | Land Improvements [Member]
Property, Plant and Equipment [Line Items]
Gross Proceeds (in thousands)		$ 4,000
2019 sales | Disposal Group, Not Discontinued Operations | Arizona | Acres of Developable Land
Property, Plant and Equipment [Line Items]
Acres of Developable Land (in acres) | a		8.8		8.8
2019 sales | Disposal Group, Not Discontinued Operations | Southeastern PA | Acres of Developable Land
Property, Plant and Equipment [Line Items]
Acres of Developable Land (in acres) | a		18.0		18.0
2019 sales | Disposal Group, Not Discontinued Operations | New Jersey | Acres of Developable Land
Property, Plant and Equipment [Line Items]
Acres of Developable Land (in acres) | a		68.4		68.4
2019 sales | Discontinued operations | Philadelphia
Property, Plant and Equipment [Line Items]
Debt Instrument, Interest Rate During Period	4.84%
Loss on debt extinguishment	$ 7,600
2019 sales | Discontinued operations | Arizona | Land, Buildings and Improvements
Property, Plant and Equipment [Line Items]
Gross Proceeds (in thousands)				$ 11,500
2019 sales | Discontinued operations | Southeastern PA | Building and Building Improvements
Property, Plant and Equipment [Line Items]
Number of buildings (in buildings) | bldg		5		5
Leaseable Square Feet (in sqft) | ft²		254,331		254,331
2019 sales | Discontinued operations | Southeastern PA | Land, Buildings and Improvements
Property, Plant and Equipment [Line Items]
Gross Proceeds (in thousands)				$ 40,200
2019 sales | Discontinued operations | New Jersey | Land, Buildings and Improvements
Property, Plant and Equipment [Line Items]
Gross Proceeds (in thousands)				$ 8,070
2019 sales | Discontinued operations | DC Metro | Building and Building Improvements
Property, Plant and Equipment [Line Items]
Number of buildings (in buildings) | bldg		2		2
Leaseable Square Feet (in sqft) | ft²		437,234		437,234
2019 sales | Discontinued operations | DC Metro | Land, Buildings and Improvements
Property, Plant and Equipment [Line Items]
Gross Proceeds (in thousands)				$ 154,250
2019 sales | Operating Segments | Disposal Group, Not Discontinued Operations | Carolinas/Richmond | Building and Building Improvements
Property, Plant and Equipment [Line Items]
Number of buildings (in buildings) | bldg		1		1
Leaseable Square Feet (in sqft) | ft²		136,000		136,000
2019 sales | Operating Segments | Disposal Group, Not Discontinued Operations | Carolinas/Richmond | Land, Buildings and Improvements
Property, Plant and Equipment [Line Items]
Gross Proceeds (in thousands)				$ 14,100