Summary of Critical and Significant Accounting Policies	12 Months Ended
Dec. 31, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Summary of Critical and Significant Accounting Policies	Critical Accounting Policies and Estimates Recognition of Goodwill and Acquired Intangible Assets — At the acquisition date, the Company records all tangible and intangible assets acquired and liabilities assumed in a business combination at fair value, the most significant of which would be goodwill and acquired intangible assets. Acquisition-date fair value represents the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants as measured on the acquisition date. Fair values were derived from valuations based on information that existed as of the acquisition date. The fair value assigned to identifiable intangible assets is primarily determined using estimates including future cash flows, discount rates, royalty rates and income tax rates utilized in a discounted cash flow model, which is a non-recurring fair value measurement based on unobservable inputs (Level 3 inputs). Acquired identified intangible assets typically include customer and contractual relationships, contractor relationships, contract backlog, non-compete agreements and trademarks. In an acquisition, the excess amount of the purchase consideration paid over the fair value of the net assets acquired and liabilities assumed is recorded as goodwill. Goodwill represents the acquired assembled workforce, potential new customers and future cash flows after the acquisition. During the measurement period, which does not exceed one year from the acquisition date, provisional amounts may be adjusted to reflect new information the Company has subsequently obtained regarding facts and circumstances that existed as of the acquisition date. Such fair value assessments require judgments and estimates, which may cause final amounts to differ materially from original estimates. Recoverability of Goodwill and Acquired Intangible Assets — Goodwill is evaluated for impairment annually, or more frequently if an event occurs or circumstances change, including but not limited to a significant decrease in expected revenues or cash flows; an adverse change in the business environment, regulatory environment or legal factors; or a substantial sustained decline in the market capitalization of our stock. Goodwill is tested at the reporting unit level, which is generally an operating segment or one level below the operating segment level, where a business operates and for which discrete financial information is available and reviewed by segment management. The Company performs its annual impairment assessment as of October 31st for each of its reporting units. When evaluating goodwill for impairment, the Company may first perform a qualitative assessment to determine whether it is more likely than not that a reporting unit is impaired. If the Company decides not to perform a qualitative assessment, or if it determines that it is more likely than not that the carrying amount of a reporting unit exceeds its fair value, a quantitative assessment is performed to determine the estimated fair value of the reporting unit. If the carrying amount of the reporting unit exceeds the estimated fair value, an impairment charge is recorded to reduce the carrying amount to its estimated fair value. The decision to perform a qualitative impairment assessment in a given year is influenced by a number of factors including: (i) the significance of the excess of the reporting units’ estimated fair value over carrying amount at the last quantitative assessment date, (ii) the amount of time between quantitative fair value assessments and (iii) the date of acquisition. The Company's only indefinite-lived intangible assets are trademarks. The Company performs its annual impairment assessment for its trademarks as of October 31st. A qualitative assessment is performed for trademarks to determine if there are any indicators that the carrying amount might not be recovered. A quantitative analysis may be performed in order to test the trademarks for impairment. If a quantitative analysis is necessary, an income approach, specifically a relief-from-royalty method, is used to estimate the fair value of the trademarks. The estimated fair value of each trademark is compared with its carrying amount to determine if impairment exists. If the carrying amount of a trademark exceeds the estimated fair value, an impairment charge would be recorded to reduce the carrying amount of the trademark. The Company performed a qualitative assessment for the October 31, 2021 annual impairment test for all of its reporting units. The Company determined there were no indicators of impairment and it was more likely than not that the fair value of each of the three reporting units exceeded its respective carrying amount by reviewing (i) macroeconomic, industry and market conditions; (ii) cost factors; (iii) overall financial performance compared with prior projections; (iv) the excess of fair value over carrying value as of the most recent quantitative assessment performed and (v) other relevant entity-specific events. Significant Accounting Policies Revenue Recognition — Revenues are recognized as control of the promised service is transferred to customers, in an amount that reflects the consideration expected in exchange for the services. The Company recognizes revenues on a gross basis as it acts as a principal for all of its revenue transactions. The Company has direct contractual relationships with its customers, bears the risks and rewards of its arrangements, has the discretion to select the contract professionals and establish the price for the services to be provided. The majority of the Company's services are provided under time-and-materials ("T&M") contracts where payments are based on fixed hourly rates for each direct labor hour expended and reimbursements for allowable material costs and out-of-pocket expenses. Revenues for T&M contracts are recognized over time, based on hours worked, because the customer simultaneously receives and consumes the benefits as services are provided. Generally, the performance of the requested service over time is a single performance obligation. To the extent actual direct labor and associated costs vary in relation to the agreed upon billing rates, the generated profit may vary. The Federal Government Segment also provides services under cost reimbursable and firm-fixed-price ("FFP") contracts, which are recognized over time based on the amount invoiced as those amounts directly correspond with the value received by a customer. Generally, these contracts contain a single performance obligation involving a significant integration of various activities that are performed together to deliver a combined service or solution. Cost reimbursable contracts are usually subject to lower risk and tend to have lower margins. From time to time, the Company may have FFP contracts in which revenues are recognized using a cost-to-cost measurement method. Under certain commercial contracts, customers may receive discounts (e.g., volume discounts, rebates, prompt-pay discounts) and adjustments to the amounts billed, which are considered variable consideration. Volume discounts are the largest component of variable consideration and are estimated using (i) the most likely amount method, (ii) contract terms and (iii) estimates of revenue. Revenues are recognized net of variable consideration to the extent it is probable a significant reversal of revenues will not occur in subsequent periods. The Company includes billable expenses (allowable material costs and out-of-pocket reimbursable expenses) in revenues and the associated expenses are included in costs of services. There are no incremental contract costs to obtain contracts. Contract fulfillment costs include, but are not limited to, direct labor for both employees and subcontractors, allowable materials such as third-party hardware and software that are integrated as part of the overall services and solutions provided to customers and out-of-pocket reimbursable expenses. Contract fulfillment costs are expensed as incurred, except for certain set-up costs for a federal government project, which were capitalized and are being amortized over the expected period of benefit. The Company’s contracts have termination for convenience provisions and do not have substantive termination penalties. Therefore, the contract duration for accounting purposes may be less than the stated terms. For accounting purposes, the Company's contracts with customers are considered to be of a short-term nature (one year or less). The Company does not disclose the value of remaining performance obligations for short-term contracts. The Company has contract liabilities for payments received in advance of providing services under certain contracts. Contract liabilities for advance payments were $13.3 million and $18.4 million at December 31, 2021 and 2020, respectively. Contract liabilities are included in other current liabilities in the accompanying consolidated balance sheets and are generally recognized as revenues within three months from the balance sheet date. Payment terms vary and the time between invoicing and when payment is due is not significant. There are no financing components to the Company’s arrangements. Costs of Services — Costs of services include direct costs consisting primarily of payroll, payroll taxes and benefit costs for the Company’s contract professionals. Costs of services also include other direct costs and reimbursable out-of-pocket expenses. Stock-Based Compensation — Stock-based compensation expense is measured based on the grant-date fair value of the respective awards and recognized over the requisite service period, net of an estimated forfeiture rate. Amortization of Finite-Lived Intangible Assets — Finite-lived intangible assets are amortized over their useful lives and are tested for recoverability whenever events or changes in circumstances indicate that the carrying amount may not be recoverable. Customer and contractual relationships and contract backlog are amortized based on the annual cash flows observed in the valuation of the asset, which generally accelerates the amortization into the earlier years reflective of the economic life of the asset. Contractor relationships and non-compete agreements are amortized using the straight-line method. Income Taxes — Income taxes are accounted for using the liability method. Deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date. Deferred tax assets are reduced by a valuation allowance if it is more likely than not that a portion of the deferred tax asset will not be realized. The Company reviews its uncertain tax positions regularly. An uncertain tax position represents the Company’s expected treatment of a tax position taken in a filed return, or planned to be taken in a future tax return or claim that has not been reflected in measuring income tax expense for financial reporting purposes. The Company recognizes the tax benefit from an uncertain tax position when it is more-likely-than- not that the position will be sustained upon examination on the basis of the technical merits or the statute of limitations for the relevant taxing authority to examine and challenge the tax position has expired. Foreign Currency Translation — The functional currency of the Company’s foreign operations is their local currency. Assets and liabilities are translated into U.S. dollars at the rate of exchange in effect on the balance sheet date. Revenues and expenses are translated at the average rates of exchange prevailing during each monthly period. The related translation adjustments are recorded as cumulative foreign currency translation adjustments in accumulated other comprehensive (loss) income as a separate component of stockholders’ equity. Cash and Cash Equivalents — The Company considers all highly liquid investments with original maturities of three months or less to be cash equivalents. Accounts Receivable Allowances — The Company estimates an allowance for expected credit losses (the inability of customers to make required payments). These estimates are based on (i) a combination of past experience and current trends, (ii) consideration of the current aging of receivables and (iii) a specific review for potential bad debts. The resulting bad debt expense is included in SG&A expenses in the accompanying consolidated statements of operations and comprehensive income. Receivables are written off when deemed uncollectible. The accounts receivable allowance was $3.1 million and $3.9 million at December 31, 2021 and 2020, respectively. Leases — The Company has operating leases for corporate offices, branch offices and data centers, which have lease terms ranging from two months to 11 years. At the inception of a contract, the Company determines if the contract contains a lease. A contract contains a lease if it conveys the right to control the use of an identified asset for a period of time in exchange for consideration. Operating lease ROU assets and lease liabilities are recognized at the lease commencement date, based on the present value of the future minimum lease payments. The Company’s leases do not provide an implicit rate of return. Therefore, the Company uses its incremental borrowing rate ("IBR") in determining the present value of lease payments. In determining the IBR, the Company considers its credit rating and the current market interest rates. The IBR approximates the interest rate the Company would pay on collateralized debt with similar terms and payments as the lease agreements and in a similar economic environment where the leased assets are located. Leases with an initial term of 12 months or less are not recorded on the balance sheet. The Company does not have finance leases. Lease expense is recognized on a straight-line basis over the lease term and is primarily included in SG&A expenses in the accompanying consolidated statements of operations and comprehensive income. Some lease agreements offer renewal options, which are assessed against relevant economic factors to determine whether it is reasonably certain that these renewal options will be exercised. As a result of this assessment, for most leases, renewal options were excluded from the minimum lease payments when calculating the operating lease assets and liabilities, as the Company does not consider the exercise of such options to be reasonably certain. The Company has lease agreements with lease and non-lease components, which are accounted for as a single lease component for all underlying asset classes. Some leases require variable payments for common area maintenance, property taxes, parking, insurance and other variable costs. The variable portion of lease payments is not included in operating lease assets or liabilities. Variable lease costs are expensed when incurred. Property and Equipment — Property and equipment are stated at cost. Depreciation and amortization are provided using the straight-line method over the estimated useful lives of the related assets, generally three to five years. Leasehold improvements are amortized over the shorter of the life of the related asset or the remaining term of the lease. Costs associated with customized internal-use software systems that have reached the application development stage and meet recoverability tests are capitalized and include external direct costs utilized in developing or obtaining the applications and payroll and payroll-related expenses for employees who are directly associated with the application development. Impairment or Disposal of Long-Lived Assets — The Company evaluates long-lived assets, other than goodwill and identifiable intangible assets with indefinite lives, for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. An impairment loss is recognized when the sum of the undiscounted future cash flows is less than the carrying amount of the asset, in which case a write down is recorded to reduce the related asset to its estimated fair value. There were no significant impairments of long-lived assets in 2021, 2020 and 2019. Workers’ Compensation Loss Reserves — The Company carries retention policies for its workers’ compensation liability exposures. Under these policies, the Company pays a base premium plus actual losses incurred, not to exceed certain stop-loss limits. The Company is insured for losses above these limits. The Company estimates its workers' compensation loss reserves based on a third-party actuarial study based on claims filed and claims incurred but not reported. The Company accounts for claims incurred but not yet reported based on estimates derived from historical claims experience and current trends of industry data. Changes in estimates, differences in estimates and actual payments for claims are recognized in the period when the estimate changed or the payment was made. Contingencies — The Company records an estimated loss from a loss contingency when information available prior to issuance of its financial statements indicates it is probable that an asset has been impaired or a liability has been incurred at the date of the financial statements and the amount of the loss can be reasonably estimated. Accounting for contingencies, such as legal settlements and workers’ compensation matters, requires the Company to use judgment. Concentration of Credit Risk — Financial instruments that potentially subject the Company to credit risks consist primarily of cash and cash equivalents and trade receivables. The Company places its cash and cash equivalents with high-quality financial institutions. Concentration of credit risk with respect to accounts receivable for the Commercial Segment is limited because of the large number of clients and their dispersion across different industries and geographies, thus spreading the trade credit risk. The Company performs ongoing credit evaluations to identify risks and maintains an allowance to address these risks. Accounts receivables for the Federal Government Segment are primarily from the U.S. government and are considered to have low credit risk.Earnings per Share — Basic earnings per share are computed using the weighted-average number of shares outstanding and diluted earnings per share are computed using the weighted-average number of shares and dilutive share equivalents (consisting of non-qualified stock options, restricted stock units and employee stock purchase plan contributions) outstanding during the periods using the treasury-stock method.
General	Basis of Presentation — The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America ("GAAP") and the rules of the Securities and Exchange Commission ("SEC"). The consolidated financial statements include the accounts of ASGN Incorporated and its wholly-owned subsidiaries ("ASGN" or the "Company"). The results of operations for acquired companies are included in the consolidated results of the Company from the date of acquisition (see Note 6. Acquisitions). All intercompany accounts and transactions have been eliminated. Certain prior period amounts have been reclassified to conform to current period presentation. In addition, retrospective reclassifications have been made to prior period consolidated financial statements and disclosures to present the Oxford Global Resources business unit (the "Oxford business") as discontinued operations (see Note 4. Discontinued Operations). Unless otherwise noted, amounts and disclosures included herein relate to our continuing operations. Use of Estimates — The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Those estimates determined to be most critical to the preparation of the financial statements are discussed below in Note 2. Summary of Critical and Significant Accounting Policies — Critical Accounting Policies and Estimates. Actual results could differ from those estimates.

Recognition of Goodwill and Acquired Intangible Assets — At the acquisition date, the Company records all tangible and intangible assets acquired and liabilities assumed in a business combination at fair value, the most significant of which would be goodwill and acquired intangible assets. Acquisition-date fair value represents the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants as measured on the acquisition date. Fair values were derived from valuations based on information that existed as of the acquisition date. The fair value assigned to identifiable intangible assets is primarily determined using estimates including future cash flows, discount rates, royalty rates and income tax rates utilized in a discounted cash flow model, which is a non-recurring fair value measurement based on unobservable inputs (Level 3 inputs). Acquired identified intangible assets typically include customer and contractual relationships, contractor relationships, contract backlog, non-compete agreements and trademarks. In an acquisition, the excess amount of the purchase consideration paid over the fair value of the net assets acquired and liabilities assumed is recorded as goodwill. Goodwill represents the acquired assembled workforce, potential new customers and future cash flows after the acquisition. During the measurement period, which does not exceed one year from the acquisition date, provisional amounts may be adjusted to reflect new information the Company has subsequently obtained regarding facts and circumstances that existed as of the acquisition date. Such fair value assessments require judgments and estimates, which may cause final amounts to differ materially from original estimates.

Recoverability of Goodwill and Acquired Intangible Assets — Goodwill is evaluated for impairment annually, or more frequently if an event occurs or circumstances change, including but not limited to a significant decrease in expected revenues or cash flows; an adverse change in the business environment, regulatory environment or legal factors; or a substantial sustained decline in the market capitalization of our stock. Goodwill is tested at the reporting unit level, which is generally an operating segment or one level below the operating segment level, where a business operates and for which discrete financial information is available and reviewed by segment management. The Company performs its annual impairment assessment as of October 31st for each of its reporting units. When evaluating goodwill for impairment, the Company may first perform a qualitative assessment to determine whether it is more likely than not that a reporting unit is impaired. If the Company decides not to perform a qualitative assessment, or if it determines that it is more likely than not that the carrying amount of a reporting unit exceeds its fair value, a quantitative assessment is performed to determine the estimated fair value of the reporting unit. If the carrying amount of the reporting unit exceeds the estimated fair value, an impairment charge is recorded to reduce the carrying amount to its estimated fair value. The decision to perform a qualitative impairment assessment in a given year is influenced by a number of factors including: (i) the significance of the excess of the reporting units’ estimated fair value over carrying amount at the last quantitative assessment date, (ii) the amount of time between quantitative fair value assessments and (iii) the date of acquisition.

The Company's only indefinite-lived intangible assets are trademarks. The Company performs its annual impairment assessment for its trademarks as of October 31st. A qualitative assessment is performed for trademarks to determine if there are any indicators that the carrying amount might not be recovered. A quantitative analysis may be performed in order to test the trademarks for impairment. If a quantitative analysis is necessary, an income approach, specifically a relief-from-royalty method, is used to estimate the fair value of the trademarks. The estimated fair value of each trademark is compared with its carrying amount to determine if impairment exists. If the carrying amount of a trademark exceeds the estimated fair value, an impairment charge would be recorded to reduce the carrying amount of the trademark.

The Company performed a qualitative assessment for the October 31, 2021 annual impairment test for all of its reporting units. The Company determined there were no indicators of impairment and it was more likely than not that the fair value of each of the three reporting units exceeded its respective carrying amount by reviewing (i) macroeconomic, industry and market conditions; (ii) cost factors; (iii) overall financial performance compared with prior projections; (iv) the excess of fair value over carrying value as of the most recent quantitative assessment performed and (v) other relevant entity-specific events.

Revenue Recognition — Revenues are recognized as control of the promised service is transferred to customers, in an amount that reflects the consideration expected in exchange for the services. The Company recognizes revenues on a gross basis as it acts as a principal for all of its revenue transactions. The Company has direct contractual relationships with its customers, bears the risks and rewards of its arrangements, has the discretion to select the contract professionals and establish the price for the services to be provided.

The majority of the Company's services are provided under time-and-materials ("T&M") contracts where payments are based on fixed hourly rates for each direct labor hour expended and reimbursements for allowable material costs and out-of-pocket expenses. Revenues for T&M contracts are recognized over time, based on hours worked, because the customer simultaneously receives and consumes the benefits as services are provided. Generally, the performance of the requested service over time is a single performance obligation. To the extent actual direct labor and associated costs vary in relation to the agreed upon billing rates, the generated profit may vary.

The Federal Government Segment also provides services under cost reimbursable and firm-fixed-price ("FFP") contracts, which are recognized over time based on the amount invoiced as those amounts directly correspond with the value received by a customer. Generally, these contracts contain a single performance obligation involving a significant integration of various activities that are performed together to deliver a combined service or solution. Cost reimbursable contracts are usually subject to lower risk and tend to have lower margins. From time to time, the Company may have FFP contracts in which revenues are recognized using a cost-to-cost measurement method.

Under certain commercial contracts, customers may receive discounts (e.g., volume discounts, rebates, prompt-pay discounts) and adjustments to the amounts billed, which are considered variable consideration. Volume discounts are the largest component of variable consideration and are estimated using (i) the most likely amount method, (ii) contract terms and (iii) estimates of revenue. Revenues are recognized net of variable consideration to the extent it is probable a significant reversal of revenues will not occur in subsequent periods. The Company includes billable expenses (allowable material costs and out-of-pocket reimbursable expenses) in revenues and the associated expenses are included in costs of services.

There are no incremental contract costs to obtain contracts. Contract fulfillment costs include, but are not limited to, direct labor for both employees and subcontractors, allowable materials such as third-party hardware and software that are integrated as part of the overall services and solutions provided to customers and out-of-pocket reimbursable expenses. Contract fulfillment costs are expensed as incurred, except for certain set-up costs for a federal government project, which were capitalized and are being amortized over the expected period of benefit.

The Company’s contracts have termination for convenience provisions and do not have substantive termination penalties. Therefore, the contract duration for accounting purposes may be less than the stated terms. For accounting purposes, the Company's contracts with customers are considered to be of a short-term nature (one year or less). The Company does not disclose the value of remaining performance obligations for short-term contracts.

The Company has contract liabilities for payments received in advance of providing services under certain contracts. Contract liabilities for advance payments were $13.3 million and $18.4 million at December 31, 2021 and 2020, respectively. Contract liabilities are included in other current liabilities in the accompanying consolidated balance sheets and are generally recognized as revenues within three months from the balance sheet date.

Amortization of Finite-Lived Intangible Assets — Finite-lived intangible assets are amortized over their useful lives and are tested for recoverability whenever events or changes in circumstances indicate that the carrying amount may not be recoverable. Customer and contractual relationships and contract backlog are amortized based on the annual cash flows observed in the valuation of the asset, which generally accelerates the amortization into the earlier years reflective of the economic life of the asset. Contractor relationships and non-compete agreements are amortized using the straight-line method.

Income Taxes — Income taxes are accounted for using the liability method. Deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date. Deferred tax assets are reduced by a valuation allowance if it is more likely than not that a portion of the deferred tax asset will not be realized.

The Company reviews its uncertain tax positions regularly. An uncertain tax position represents the Company’s expected treatment of a tax position taken in a filed return, or planned to be taken in a future tax return or claim that has not been reflected in measuring income tax expense for financial reporting purposes. The Company recognizes the tax benefit from an uncertain tax position when it is more-likely-than-

Foreign Currency Translation — The functional currency of the Company’s foreign operations is their local currency. Assets and liabilities are translated into U.S. dollars at the rate of exchange in effect on the balance sheet date. Revenues and expenses are translated at the average rates of exchange prevailing during each monthly period. The related translation adjustments are recorded as cumulative foreign currency translation adjustments in accumulated other comprehensive (loss) income as a separate component of stockholders’ equity.

Accounts Receivable Allowances — The Company estimates an allowance for expected credit losses (the inability of customers to make required payments). These estimates are based on (i) a combination of past experience and current trends, (ii) consideration of the current aging of receivables and (iii) a specific review for potential bad debts. The resulting bad debt expense is included in SG&A expenses in the accompanying consolidated statements of operations and comprehensive income. Receivables are written off when deemed uncollectible. The accounts receivable allowance was $3.1 million and $3.9 million at December 31, 2021 and 2020, respectively.

Leases — The Company has operating leases for corporate offices, branch offices and data centers, which have lease terms ranging from two months to 11 years. At the inception of a contract, the Company determines if the contract contains a lease. A contract contains a lease if it conveys the right to control the use of an identified asset for a period of time in exchange for consideration. Operating lease ROU assets and lease liabilities are recognized at the lease commencement date, based on the present value of the future minimum lease payments. The Company’s leases do not provide an implicit rate of return. Therefore, the Company uses its incremental borrowing rate ("IBR") in determining the present value of lease payments. In determining the IBR, the Company considers its credit rating and the current market interest rates. The IBR approximates the interest rate the Company would pay on collateralized debt with similar terms and payments as the lease agreements and in a similar economic environment where the leased assets are located. Leases with an initial term of 12 months or less are not recorded on the balance sheet. The Company does not have finance leases.

Lease expense is recognized on a straight-line basis over the lease term and is primarily included in SG&A expenses in the accompanying consolidated statements of operations and comprehensive income. Some lease agreements offer renewal options, which are assessed against relevant economic factors to determine whether it is reasonably certain that these renewal options will be exercised. As a result of this assessment, for most leases, renewal options were excluded from the minimum lease payments when calculating the operating lease assets and liabilities, as the Company does not consider the exercise of such options to be reasonably certain.

The Company has lease agreements with lease and non-lease components, which are accounted for as a single lease component for all underlying asset classes. Some leases require variable payments for common area maintenance, property taxes, parking, insurance and other variable costs. The variable portion of lease payments is not included in operating lease assets or liabilities. Variable lease costs are expensed when incurred.

Property and Equipment — Property and equipment are stated at cost. Depreciation and amortization are provided using the straight-line method over the estimated useful lives of the related assets, generally three to five years. Leasehold improvements are amortized over the shorter of the life of the related asset or the remaining term of the lease. Costs associated with customized internal-use software systems that have reached the application development stage and meet recoverability tests are capitalized and include external direct costs utilized in developing or obtaining the applications and payroll and payroll-related expenses for employees who are directly associated with the application development.

Impairment or Disposal of Long-Lived Assets — The Company evaluates long-lived assets, other than goodwill and identifiable intangible assets with indefinite lives, for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. An impairment loss is recognized when the sum of the undiscounted future cash flows is less than the carrying amount of the asset, in which case a write down is recorded to reduce the related asset to its estimated fair value. There were no significant impairments of long-lived assets in 2021, 2020 and 2019.

Workers’ Compensation Loss Reserves — The Company carries retention policies for its workers’ compensation liability exposures. Under these policies, the Company pays a base premium plus actual losses incurred, not to exceed certain stop-loss limits. The Company is insured for losses above these limits. The Company estimates its workers' compensation loss reserves based on a third-party actuarial study based on claims filed and claims incurred but not reported. The Company accounts for claims incurred but not yet reported based on estimates derived from historical claims experience and current trends of industry data. Changes in estimates, differences in estimates and actual payments for claims are recognized in the period when the estimate changed or the payment was made.

Contingencies — The Company records an estimated loss from a loss contingency when information available prior to issuance of its financial statements indicates it is probable that an asset has been impaired or a liability has been incurred at the date of the financial statements and the amount of the loss can be reasonably estimated. Accounting for contingencies, such as legal settlements and workers’ compensation matters, requires the Company to use judgment.