Commitment and Contingencies - Additional Information (Detail) $ in Thousands			1 Months Ended		3 Months Ended		9 Months Ended
	Nov. 06, 2020 Property	Nov. 05, 2020 USD ($) Property	Sep. 30, 2020 USD ($) Property	Mar. 31, 2020 USD ($) Property	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) Property	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Commitment And Contingencies [Line Items]
Operating lease, aggregate right-of-use assets			$ 20,600		$ 20,600		$ 20,600		$ 20,200
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]			us-gaap:OtherAssetsMember		us-gaap:OtherAssetsMember		us-gaap:OtherAssetsMember		us-gaap:OtherAssetsMember
Operating lease liabilities			$ 20,200		$ 20,200		$ 20,200		$ 19,900
Operating Lease, Liability, Statement of Financial Position [Extensible List]			us-gaap:AccountsPayableAndAccruedLiabilitiesMember		us-gaap:AccountsPayableAndAccruedLiabilitiesMember		us-gaap:AccountsPayableAndAccruedLiabilitiesMember		us-gaap:AccountsPayableAndAccruedLiabilitiesMember
Operating lease expenses					$ 500	$ 600	$ 1,600	$ 1,900
Weighted average remaining lease term			11 years 8 months 12 days		11 years 8 months 12 days		11 years 8 months 12 days
Weighted average discount rate			4.66%		4.66%		4.66%
Operating lease commitments			$ 26,570		$ 26,570		$ 26,570
Number of storage facilities acquired | Property							31
Expansion and enhancement contracts			16,000
Subsequent Event [Member]
Commitment And Contingencies [Line Items]
Number of storage facilities acquired | Property	5
Self-storage Facilities [Member]
Commitment And Contingencies [Line Items]
Purchase price of business to be acquired			$ 37,900
Number of storage facilities acquired | Property			3
Number of storage facilities to be sold | Property				1
Sale price of storage facilities				$ 19,000
Self-storage Facilities [Member] | Subsequent Event [Member]
Commitment And Contingencies [Line Items]
Purchase price of business to be acquired		$ 13,700
Number of storage facilities acquired | Property		1