Real Estate Facilities (Narrative) (Details) $ in Thousands				12 Months Ended
	Oct. 25, 2018 USD ($)	Oct. 18, 2018 USD ($)	Dec. 31, 2017 USD ($) ft² item	Dec. 31, 2019 USD ($) ft² item	Dec. 31, 2018 USD ($) ft² item	Dec. 31, 2017 USD ($) ft² item	Dec. 31, 2016 USD ($)
Schedule Of Real Estate Facilities [Line Items]
Net rentable square feet | ft²				168,908
Cash to acquire self-storage facilities				$ 437,758	$ 181,020	$ 285,279
Gain on real estate sales				341	37,903	1,421
Other assets				174,344	131,097
Cash proceeds from sale of real estate facilities						6,400	$ 300
Payments for development and expansion of real estate facilities				284,700	340,000	337,300
Payments for development and expansion of real estate facilities for: Costs incurred in previous periods				81,351	58,792	48,055
Costs incurred for development and expansion of real estate facilities				235,687	362,397	349,712
Unpaid costs incurred for development and expansion of real estate facilities				32,400	81,200	60,500
Payments for capital expenditures to maintain real estate facilities				187,300	141,000	120,600
Payments for capital expenditures to maintain real estate facilities for: Costs incurred in previous periods				11,331	13,005	8,964
Costs incurred for capital expenditures to maintain real estate facilities				192,539	139,397	124,780
Unpaid costs incurred for capital expenditures to maintain real estate facilities				16,600	11,400	13,100
Transfer from other assets					9,469
Real estate facilities for federal tax purposes				$ 10,600,000
Development and Redevelopment [Member]
Schedule Of Real Estate Facilities [Line Items]
Aggregate costs to develop and expand					$ 348,300	$ 311,600
Addtional net rentable square feet | ft²					3,000,000.0	2,700,000
Disposal Of Commercial Facility [Member]
Schedule Of Real Estate Facilities [Line Items]
Gain on real estate sales	$ 4,600
Cash proceeds from sale of real estate facilities	$ 8,700
Disposal Of Real Estate Facilities [Member]
Schedule Of Real Estate Facilities [Line Items]
Gain on real estate sales					$ 1,800
Cash proceeds from sale of real estate facilities					$ 3,400
Acquisition Of Self-Storage Facilities Other Investments [Member]
Schedule Of Real Estate Facilities [Line Items]
Number of operating self-storage facilities | item			12	44	25
Net rentable square feet | ft²			749,000	3,133,000	1,600,000
Total cost of acquisition			$ 141,800	$ 439,600
Cash to acquire self-storage facilities			$ 135,500	437,800	$ 181,000
Ownership interest, percentage			74.25%			74.25%
Mortgage debt assumed				1,800
Allocated to intangible assets			$ 9,000	$ 18,500	$ 11,600	$ 9,000
Other assets			300			$ 300
Value of existing investment			6,300
Acquisition Of Self-Storage Facilities From Third Parties [Member]
Schedule Of Real Estate Facilities [Line Items]
Number of operating self-storage facilities | item						22
Net rentable square feet | ft²						1,365,000
Cash to acquire self-storage facilities						$ 149,800
Allocated to intangible assets			$ 8,200			$ 8,200
Acquisition Of Commercial Facility [Member]
Schedule Of Real Estate Facilities [Line Items]
Number of operating self-storage facilities | item				1
Net rentable square feet | ft²				46,000
Newly Developed and Expansion Projects [Member] | Completed Developed and Expansion Project [Member]
Schedule Of Real Estate Facilities [Line Items]
Aggregate costs to develop and expand				$ 379,100
Addtional net rentable square feet | ft²				3,700,000
West London [Member] | Disposal Of Real Estate [Member]
Schedule Of Real Estate Facilities [Line Items]
Gain on real estate sales		$ 31,500
Cash proceeds from sale of real estate facilities		42,100
Cumulative other comprehensive loss, foreign currency translation		$ 4,800