Basis of Presentation and Summary of Significant Accounting Policies (Policy)	12 Months Ended
Dec. 31, 2021
Accounting Policies [Abstract]
Basis of Presentation	Basis of PresentationThe consolidated financial statements are presented on an accrual basis in accordance with U.S. generally accepted accounting principles (“GAAP”) as set forth in the Accounting Standards Codification of the Financial Accounting Standards Board (“FASB”), and in conformity with the rules and regulations of the Securities and Exchange Commission (“SEC”).
Consolidation and Equity Method of Accounting	Consolidation and Equity Method of Accounting We consider entities to be Variable Interest Entities (“VIEs”) when they have insufficient equity to finance their activities without additional subordinated financial support provided by other parties, or the equity holders as a group do not have a controlling financial interest. In addition, we have general partner interests in limited partnerships along with third-party investors to develop, construct or operate self-storage facilities. As the general partner, we consider the limited partnerships to be VIEs if the limited partners lack both substantive participating rights and substantive kick-out rights. We consolidate VIEs when we have (i) the power to direct the activities most significantly impacting economic performance, and (ii) either the obligation to absorb losses or the right to receive benefits from the VIE. The total assets, primarily real estate assets, and the total liabilities of our consolidated VIEs are not material as of December 31, 2021. We consolidate all other entities when we control them through voting shares or contractual rights. We refer to the entities we consolidate, for the period in which the reference applies, collectively as the “Subsidiaries,” and we eliminate intercompany transactions and balances. We account for our investments in entities that we do not consolidate but over which we have significant influence using the equity method of accounting. We refer to these entities, for the periods in which the reference applies, collectively as the “Unconsolidated Real Estate Entities,” and we eliminate intra-entity profits and losses and amortize any differences between the cost of our investment and the underlying equity in net assets against equity in earnings as if the Unconsolidated Real Estate Entity were a consolidated subsidiary. Equity in earnings of unconsolidated real estate entities presented on our income statements represents our pro-rata share of the earnings of the Unconsolidated Real Estate Entities. The dividends we receive from the Unconsolidated Real Estate Entities are reflected on our consolidated statements of cash flows as “distributions from cumulative equity in earnings of unconsolidated real estate entities” to the extent of our cumulative equity in earnings, with any excess classified as “distributions in excess of cumulative equity in earnings from unconsolidated real estate entities.”
Use of Estimates	Use of Estimates The preparation of consolidated financial statements and accompanying notes in conformity with GAAP requires us to make estimates and assumptions that affect the amounts reported. Actual results could differ from those estimates and assumptions.
Cash Equivalents, Restricted Cash and Marketable Securities	Cash Equivalents and Restricted Cash Cash equivalents represent highly liquid financial instruments that mature within three months of acquisition such as money market funds with a rating of at least AAA by Standard & Poor's, commercial paper that is rated A1 by Standard & Poor's or deposits with highly rated commercial banks. Restricted cash, which represent amounts used to collateralize our insurance obligations and are restricted from general corporate use, are included in other assets.
Fair Value	Fair Value As used herein, the term “fair value” is the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. In the absence of active markets for identical assets or liabilities, such measurements involve developing assumptions based on market observable data and, in the absence of such data, internal information that is consistent with what market participants would use in a hypothetical transaction that occurs at the balance sheet date. Assets and liabilities recorded at fair value are measured and classified in accordance with a three-tier fair value hierarchy based on the observability of the inputs available in the market used to measure fair value: Level 1 Quoted prices in active markets for identical assets or liabilities at the measurement date. Level 2 Significant observable inputs other than Level 1, that are observable for the asset or liability, either directly or indirectly through corroboration with observable market data. Level 3 Unobservable inputs that are supported by little or no market data for the related assets or liabilities. The categorization of a financial instrument within the valuation hierarchy is based upon the lowest level of input that is significant to the fair value measurement. Our financial instruments consist of cash and cash equivalents, restricted cash, other assets, other liabilities, and notes payable. Cash equivalents, restricted cash, other assets and other liabilities are stated at book value, which approximates fair value as of the balance sheet date due to the short time period to maturity. We estimate and disclose the fair value of our notes payable using Level 2 inputs by discounting the related future cash flows at a rate based upon quoted interest rates for securities that have similar characteristics such as credit quality and time to maturity. We use significant judgment to estimate fair values of real estate facilities, goodwill, and other intangible assets for the purposes of purchase price allocation or impairment analysis. In estimating their values, we consider Level 3 inputs such as market prices of land, market capitalization rates, expected returns, earnings multiples, projected levels of earnings, costs of construction, and functional depreciation.
Real Estate Facilities	Real Estate Facilities We record real estate facilities at cost. We capitalize all costs incurred to acquire, develop, construct, renovate and improve facilities as part of major repair and maintenance programs, including interest and property taxes incurred during the construction period. We expense the costs of demolition of existing facilities associated with a renovation as incurred. We allocate the net acquisition cost of acquired real estate facilities to the underlying land, buildings, and identified intangible assets based upon their respective individual estimated fair values. We expense costs associated with dispositions of real estate, as well as routine repairs and maintenance costs, as incurred. We depreciate buildings and improvements on a straight-line basis over estimated useful lives ranging generally between 5 to 25 years. When we sell a full or partial interest in a real estate facility without retaining a controlling interest following sale, we recognize a gain or loss on sale as if 100% of the property was sold at fair value. If we retain a controlling interest following the sale, we record a noncontrolling interest for the book value of the partial interest sold, and recognize additional paid-in capital for the difference between the consideration received and the partial interest at book value.
Goodwill and Other Intangible Assets	Goodwill and Other Intangible Assets Intangible assets consist of goodwill, the Shurgard® trade name, which Shurgard uses pursuant to a fee-based licensing agreement, and finite-lived assets. Goodwill and the Shurgard® trade name have indefinite lives and are not amortized. Our finite-lived assets consist primarily of (i) acquired customers in place amortized relative to the benefit of the customers in place, with such amortization reflected as depreciation and amortization expense on our income statement and (ii) property tax abatements acquired and amortized relative to the reduction in property tax paid, with such amortization reflected as self-storage cost of operations on our income statement.
Evaluation of Asset Impairment	Evaluation of Asset Impairment We evaluate our real estate and finite-lived intangible assets for impairment each quarter. If there are indicators of impairment and we determine that the asset is not recoverable from future undiscounted cash flows to be received through the asset’s remaining life (or, if earlier, the expected disposal date), we record an impairment charge to the extent the carrying amount exceeds the asset’s estimated fair value or net proceeds from expected disposal. We evaluate our investments in unconsolidated real estate entities for impairment quarterly. We record an impairment charge to the extent the carrying amount exceeds estimated fair value, when we believe any such shortfall is other than temporary. We evaluate goodwill for impairment annually and whenever relevant events, circumstances, and other related factors indicate that fair value of the related reporting unit may be less than the carrying amount. If we determine that the fair value of the reporting unit exceeds the aggregate carrying amount, no impairment charge is recorded. Otherwise, we record an impairment charge to the extent the carrying amount of the goodwill exceeds the amount that would be allocated to goodwill if the reporting unit were acquired for estimated fair value. We evaluate other indefinite-lived intangible assets, such as the Shurgard® trade name for impairment at least annually and whenever relevant events, circumstances and other related factors indicate that the fair value is less than the carrying amount. When we conclude that it is likely that the asset is not impaired, we do not record an impairment charge and no further analysis is performed. Otherwise, we record an impairment charge to the extent the carrying amount exceeds the asset’s estimated fair value.
Revenue and Expense Recognition	Revenue and Expense Recognition We recognize revenues from self-storage facilities, which primarily comprise rental income earned pursuant to month-to-month leases, as well as associated late charges and administrative fees, as earned. Promotional discounts reduce rental income over the promotional period, which is generally one month. We recognize ancillary revenues when earned. We accrue for property tax expense based upon actual amounts billed and, in some circumstances, estimates when bills or assessments have not been received from the taxing authorities. If these estimates are incorrect, the timing and amount of expense recognition could be incorrect. We expense cost of operations (including advertising expenditures), general and administrative expense, and interest expense as incurred.
Foreign Currency Exchange Translation	Foreign Currency Exchange Translation The local currency (primarily the Euro) is the functional currency for our interests in foreign operations. The related balance sheet amounts are translated into U.S. Dollars at the exchange rates at the respective financial statement date, while amounts on our consolidated statements of income are translated at the average exchange rates during the respective period. Cumulative translation adjustments, to the extent not included in cumulative net income, are included in equity as a component of accumulated other comprehensive income (loss). When financial instruments denominated in a currency other than the U.S. Dollar are expected to be settled in cash in the foreseeable future, the impact of changes in the U.S. Dollar equivalent are reflected in current earnings. At December 31, 2021, due primarily to our investment in Shurgard (Note 4) and our notes payable denominated in Euros (Note 7), our operating results and financial position are affected by fluctuations in currency exchange rates between the Euro, and to a lesser extent, other European currencies, against the U.S. Dollar.
Income Taxes	Income Taxes We have elected to be treated as a REIT, as defined in the Internal Revenue Code of 1986, as amended (the “Code”). For each taxable year in which we qualify for taxation as a REIT, we will not be subject to U.S. federal corporate income tax on our “REIT taxable income” (generally, taxable income subject to specified adjustments, including a deduction for dividends paid and excluding our net capital gain) that is distributed to our shareholders. We believe we have met these REIT requirements for all periods presented herein. Accordingly, we have recorded no U.S. federal corporate income tax expense related to our REIT taxable income. Our tenant reinsurance, merchandise, and third party management operations are subject to corporate income tax and such taxes are included in general and administrative expenses. We also incur income and other taxes in certain states, which are included in general and administrative expense. We recognize tax benefits of uncertain income tax positions that are subject to audit only if we believe it is more likely than not that the position would ultimately be sustained assuming the relevant taxing authorities had full knowledge of the relevant facts and circumstances of our positions.
Share-Based Compensation	Share-Based Compensation We generally estimate the fair value of share-based payment awards on the date of grant. We determine the fair value of restricted share units ("RSUs") based on the closing market price of the Company’s common stock on the date of grant. We value stock options with no market conditions at the grant date using the Black-Scholes option-pricing model. We value stock options with market conditions at the grant date using a Monte-Carlo valuation simulation. Our determination of the fair value of share-based payment awards on the date of grant using an option-pricing model or Monte-Carlo valuation simulation is affected by our stock price as well as assumptions regarding a number of subjective and complex variables. These variables include, but are not limited to, our expected stock price volatility over the expected term of the awards and actual and projected stock option exercise behaviors. For performance-based restricted share units and stock options, we adjust compensation cost each quarter as needed for any changes in the assessment of the probability that the specified performance criteria will be achieved. We amortize the grant-date fair value of awards as compensation expense over the service period, which begins on the grant date and ends on the expected vesting date. For awards that are earned solely upon the passage of time and continued service, the entire cost of the award is amortized on a straight-line basis over the service period. For awards with market and/or performance conditions, the individual cost of each vesting is amortized separately over each individual service period (the “accelerated attribution” method). The estimated number of stock awards that will ultimately vest requires judgment, and to the extent actual results or updated estimates differ from our current estimates, such amounts will be recorded as a cumulative adjustment in the period estimates are revised. In amortizing share-based compensation expense, we do not estimate future forfeitures. Instead, we reverse previously amortized share-based compensation expense with respect to grants that are forfeited in the period the employee terminates employment. In July 2020, we modified our share-based compensation plans to allow immediate vesting upon retirement (“Retirement Acceleration”), and to extend the exercisability of outstanding stock options up to a year after retirement, for currently outstanding and future grants. Prior to the modification, unvested awards were forfeited, and outstanding vested stock options were cancelled, upon retirement. Employees are eligible for Retirement Acceleration if they meet certain conditions including length of service, age, notice of intent to retire, and facilitation of succession for their role. This modification results in accelerating amortization of compensation expense for each grant by changing the end of the service period from the original vesting date to the date an employee is expected to be eligible for Retirement Acceleration, if earlier.

Basis of Presentation and Summary of Significant Accounting Policies (Policy)

12 Months Ended

Dec. 31, 2021

Basis of PresentationThe consolidated financial statements are presented on an accrual basis in accordance with U.S. generally accepted accounting principles (“GAAP”) as set forth in the Accounting Standards Codification of the Financial Accounting Standards Board (“FASB”), and in conformity with the rules and regulations of the Securities and Exchange Commission (“SEC”).

Consolidation and Equity Method of Accounting

We consider entities to be Variable Interest Entities (“VIEs”) when they have insufficient equity to finance their activities without additional subordinated financial support provided by other parties, or the equity holders as a group do not have a controlling financial interest. In addition, we have general partner interests in limited partnerships along with third-party investors to develop, construct or operate self-storage facilities. As the general partner, we consider the limited partnerships to be VIEs if the limited partners lack both substantive participating rights and substantive kick-out rights. We consolidate VIEs when we have (i) the power to direct the activities most significantly impacting economic performance, and (ii) either the obligation to absorb losses or the right to receive benefits from the VIE. The total assets, primarily real estate assets, and the total liabilities of our consolidated VIEs are not material as of December 31, 2021. We consolidate all other entities when we control them through voting shares or contractual rights. We refer to the entities we consolidate, for the period in which the reference applies, collectively as the “Subsidiaries,” and we eliminate intercompany transactions and balances.

We account for our investments in entities that we do not consolidate but over which we have significant influence using the equity method of accounting. We refer to these entities, for the periods in which the reference applies, collectively as the “Unconsolidated Real Estate Entities,” and we eliminate intra-entity profits and losses and amortize any differences between the cost of our investment and the underlying equity in net assets against equity in earnings as if the Unconsolidated Real Estate Entity were a consolidated subsidiary.

Equity in earnings of unconsolidated real estate entities presented on our income statements represents our pro-rata share of the earnings of the Unconsolidated Real Estate Entities. The dividends we receive from the Unconsolidated Real Estate Entities are reflected on our consolidated statements of cash flows as “distributions from cumulative equity in earnings of unconsolidated real estate entities” to the extent of our cumulative equity in earnings, with any excess classified as “distributions in excess of cumulative equity in earnings from unconsolidated real estate entities.”

Use of Estimates

The preparation of consolidated financial statements and accompanying notes in conformity with GAAP requires us to make estimates and assumptions that affect the amounts reported. Actual results could differ from those estimates and assumptions.

Cash Equivalents, Restricted Cash and Marketable Securities

Cash Equivalents and Restricted Cash

Cash equivalents represent highly liquid financial instruments that mature within three months of acquisition such as money market funds with a rating of at least AAA by Standard & Poor's, commercial paper that is rated A1 by Standard & Poor's or deposits with highly rated commercial banks. Restricted cash, which represent amounts used to collateralize our insurance obligations and are restricted from general corporate use, are included in other assets.

Fair Value

As used herein, the term “fair value” is the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. In the absence of active markets for identical assets or liabilities, such measurements involve developing assumptions based on market observable data and, in the absence of such data, internal information that is consistent with what market participants would use in a hypothetical transaction that occurs at the balance sheet date.

Assets and liabilities recorded at fair value are measured and classified in accordance with a three-tier fair value hierarchy based on the observability of the inputs available in the market used to measure fair value:

Level 1 Quoted prices in active markets for identical assets or liabilities at the measurement date.

Level 2 Significant observable inputs other than Level 1, that are observable for the asset or liability, either directly or indirectly through corroboration with observable market data.

Level 3 Unobservable inputs that are supported by little or no market data for the related assets or liabilities.

The categorization of a financial instrument within the valuation hierarchy is based upon the lowest level of input that is significant to the fair value measurement.

Our financial instruments consist of cash and cash equivalents, restricted cash, other assets, other liabilities, and notes payable. Cash equivalents, restricted cash, other assets and other liabilities are stated at book value, which approximates fair value as of the balance sheet date due to the short time period to maturity.

We estimate and disclose the fair value of our notes payable using Level 2 inputs by discounting the related future cash flows at a rate based upon quoted interest rates for securities that have similar characteristics such as credit quality and time to maturity.

We use significant judgment to estimate fair values of real estate facilities, goodwill, and other intangible assets for the purposes of purchase price allocation or impairment analysis. In estimating their values, we consider Level 3 inputs such as market prices of land, market capitalization rates, expected returns, earnings multiples, projected levels of earnings, costs of construction, and functional depreciation.

Real Estate Facilities

We record real estate facilities at cost. We capitalize all costs incurred to acquire, develop, construct, renovate and improve facilities as part of major repair and maintenance programs, including interest and property taxes incurred during the construction period. We expense the costs of demolition of existing facilities associated with a renovation as incurred. We allocate the net acquisition cost of acquired real estate facilities to the underlying land, buildings, and identified intangible assets based upon their respective individual estimated fair values.

We expense costs associated with dispositions of real estate, as well as routine repairs and maintenance costs, as incurred. We depreciate buildings and improvements on a straight-line basis over estimated useful lives ranging generally between 5 to 25 years.

When we sell a full or partial interest in a real estate facility without retaining a controlling interest following sale, we recognize a gain or loss on sale as if 100% of the property was sold at fair value. If we retain a controlling interest following the sale, we record a noncontrolling interest for the book value of the partial interest sold, and recognize additional paid-in capital for the difference between the consideration received and the partial interest at book value.

Goodwill and Other Intangible Assets

Intangible assets consist of goodwill, the Shurgard® trade name, which Shurgard uses pursuant to a fee-based licensing agreement, and finite-lived assets. Goodwill and the Shurgard® trade name have indefinite lives and are not amortized. Our finite-lived assets consist primarily of (i) acquired customers in place amortized relative to the benefit of the customers in place, with such amortization reflected as depreciation and amortization expense on our income statement and (ii) property tax abatements acquired and amortized relative to the reduction in property tax paid, with such amortization reflected as self-storage cost of operations on our income statement.

Evaluation of Asset Impairment

We evaluate our real estate and finite-lived intangible assets for impairment each quarter. If there are indicators of impairment and we determine that the asset is not recoverable from future undiscounted cash flows to be received through the asset’s remaining life (or, if earlier, the expected disposal date), we record an impairment charge to the extent the carrying amount exceeds the asset’s estimated fair value or net proceeds from expected disposal.

We evaluate our investments in unconsolidated real estate entities for impairment quarterly. We record an impairment charge to the extent the carrying amount exceeds estimated fair value, when we believe any such shortfall is other than temporary.

We evaluate goodwill for impairment annually and whenever relevant events, circumstances, and other related factors indicate that fair value of the related reporting unit may be less than the carrying amount. If we determine that the fair value of the reporting unit exceeds the aggregate carrying amount, no impairment charge is recorded. Otherwise, we record an impairment charge to the extent the carrying amount of the goodwill exceeds the amount that would be allocated to goodwill if the reporting unit were acquired for estimated fair value.

We evaluate other indefinite-lived intangible assets, such as the Shurgard® trade name for impairment at least annually and whenever relevant events, circumstances and other related factors indicate that the fair value is less than the carrying amount. When we conclude that it is likely that the asset is not impaired, we do not record an impairment charge and no further analysis is performed. Otherwise, we record an impairment charge to the extent the carrying amount exceeds the asset’s estimated fair value.

Revenue and Expense Recognition

We recognize revenues from self-storage facilities, which primarily comprise rental income earned pursuant to month-to-month leases, as well as associated late charges and administrative fees, as earned. Promotional discounts

reduce rental income over the promotional period, which is generally one month. We recognize ancillary revenues when earned. We accrue for property tax expense based upon actual amounts billed and, in some circumstances, estimates when bills or assessments have not been received from the taxing authorities. If these estimates are incorrect, the timing and amount of expense recognition could be incorrect. We expense cost of operations (including advertising expenditures), general and administrative expense, and interest expense as incurred.

Foreign Currency Exchange Translation

The local currency (primarily the Euro) is the functional currency for our interests in foreign operations. The related balance sheet amounts are translated into U.S. Dollars at the exchange rates at the respective financial statement date, while amounts on our consolidated statements of income are translated at the average exchange rates during the respective period. Cumulative translation adjustments, to the extent not included in cumulative net income, are included in equity as a component of accumulated other comprehensive income (loss).

When financial instruments denominated in a currency other than the U.S. Dollar are expected to be settled in cash in the foreseeable future, the impact of changes in the U.S. Dollar equivalent are reflected in current earnings.

At December 31, 2021, due primarily to our investment in Shurgard (Note 4) and our notes payable denominated in Euros (Note 7), our operating results and financial position are affected by fluctuations in currency exchange rates between the Euro, and to a lesser extent, other European currencies, against the U.S. Dollar.

Income Taxes

We have elected to be treated as a REIT, as defined in the Internal Revenue Code of 1986, as amended (the “Code”). For each taxable year in which we qualify for taxation as a REIT, we will not be subject to U.S. federal corporate income tax on our “REIT taxable income” (generally, taxable income subject to specified adjustments, including a deduction for dividends paid and excluding our net capital gain) that is distributed to our shareholders. We believe we have met these REIT requirements for all periods presented herein. Accordingly, we have recorded no U.S. federal corporate income tax expense related to our REIT taxable income.

Our tenant reinsurance, merchandise, and third party management operations are subject to corporate income tax and such taxes are included in general and administrative expenses. We also incur income and other taxes in certain states, which are included in general and administrative expense.

We recognize tax benefits of uncertain income tax positions that are subject to audit only if we believe it is more likely than not that the position would ultimately be sustained assuming the relevant taxing authorities had full knowledge of the relevant facts and circumstances of our positions.

Share-Based Compensation

We generally estimate the fair value of share-based payment awards on the date of grant. We determine the fair value of restricted share units ("RSUs") based on the closing market price of the Company’s common stock on the date of grant. We value stock options with no market conditions at the grant date using the Black-Scholes option-pricing model. We value stock options with market conditions at the grant date using a Monte-Carlo valuation simulation. Our determination of the fair value of share-based payment awards on the date of grant using an option-pricing model or Monte-Carlo valuation simulation is affected by our stock price as well as assumptions regarding a number of subjective and complex variables. These variables include, but are not limited to, our expected stock price volatility over the expected term of the awards and actual and projected stock option exercise behaviors. For performance-based restricted share units and stock options, we adjust compensation cost each quarter as needed for any changes in the assessment of the probability that the specified performance criteria will be achieved.

We amortize the grant-date fair value of awards as compensation expense over the service period, which begins on the grant date and ends on the expected vesting date. For awards that are earned solely upon the passage of time and continued service, the entire cost of the award is amortized on a straight-line basis over the service period. For awards with market and/or performance conditions, the individual cost of each vesting is amortized separately over each individual service period (the “accelerated attribution” method). The estimated number of stock awards that will ultimately vest requires judgment, and to the extent actual results or updated estimates differ from our current estimates, such amounts will be recorded as a cumulative adjustment in the period estimates are revised. In amortizing share-based compensation expense, we do not estimate future forfeitures. Instead, we reverse previously amortized share-based compensation expense with respect to grants that are forfeited in the period the employee terminates employment.

In July 2020, we modified our share-based compensation plans to allow immediate vesting upon retirement (“Retirement Acceleration”), and to extend the exercisability of outstanding stock options up to a year after retirement, for currently outstanding and future grants. Prior to the modification, unvested awards were forfeited, and outstanding vested stock options were cancelled, upon retirement. Employees are eligible for Retirement Acceleration if they meet certain conditions including length of service, age, notice of intent to retire, and facilitation of succession for their role. This modification results in accelerating amortization of compensation expense for each grant by changing the end of the service period from the original vesting date to the date an employee is expected to be eligible for Retirement Acceleration, if earlier.