Real Estate Facilities (Activity In Real Estate Facilities) (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020 USD ($) ft² item	Dec. 31, 2019 USD ($) ft² item
Property, Plant and Equipment [Line Items]
Beginning Balances	$ 1,889,238
Accumulated Depreciation, Beginning Balances	(1,158,489)
Acquisition of real estate facility	13,276
Capital expenditures	23,358
Depreciation and amortization expense	(67,035)
Transfer to properties held for sale	30
Accumulated Depreciation, Ending Balances	(1,210,473)	$ (1,158,489)
Ending Balances	$ 1,858,867	1,889,238
Properties held for sale, net		$ 15,264
Number of buildings | item	672
Rentable area (in square feet) | ft²	27,500,000
Properties Held for Sale [Member]
Property, Plant and Equipment [Line Items]
Number of buildings | item		2
Rentable area (in square feet) | ft²		40,000
Land [Member]
Property, Plant and Equipment [Line Items]
Beginning Balances	$ 844,419
Acquisition of real estate facility	11,123
Ending Balances	855,542	$ 844,419
Buildings And Improvements [Member]
Property, Plant and Equipment [Line Items]
Beginning Balances	2,203,308
Acquisition of real estate facility	2,153
Capital expenditures	23,358
Disposals	(15,005)
Transfer to properties held for sale	(16)
Ending Balances	2,213,798	2,203,308
Accumulated Depreciation [Member]
Property, Plant and Equipment [Line Items]
Accumulated Depreciation, Beginning Balances	(1,158,489)
Accumulated Depreciation, Disposals	15,005
Depreciation and amortization expense	(67,035)
Transfer to properties held for sale	46
Accumulated Depreciation, Ending Balances	(1,210,473)	(1,158,489)
Reclassification [Member]
Property, Plant and Equipment [Line Items]
Beginning Balances	(3,800)
Ending Balances		(3,800)
Properties held for sale, net		3,800
Reclassification [Member] | Land [Member]
Property, Plant and Equipment [Line Items]
Beginning Balances	(2,200)
Ending Balances		(2,200)
Properties held for sale, net		2,200
Reclassification [Member] | Buildings And Improvements [Member]
Property, Plant and Equipment [Line Items]
Beginning Balances	(2,800)
Ending Balances		(2,800)
Properties held for sale, net		2,800
Reclassification [Member] | Accumulated Depreciation [Member]
Property, Plant and Equipment [Line Items]
Beginning Balances	$ (1,200)
Ending Balances		(1,200)
Properties held for sale, net		$ 1,200