Summary of Significant Accounting Policies	12 Months Ended
Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies Principles of Consolidation. The consolidated financial statements include the accounts of ASGN Incorporated and its wholly-owned subsidiaries (the “Company”). All intercompany accounts and transactions have been eliminated. Use of Estimates. The preparation of financial statements in conformity with generally accepted accounting principles 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. Actual results could differ from those estimates. 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. Revenues are recognized net of variable consideration to the extent it is probable a significant reversal of revenues will not occur in subsequent periods, see Note 11 – Revenues. Costs of Services. Costs of services include direct costs of contract assignments 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. 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, and as such, their 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 income (loss) as a separate component of stockholders’ equity. Gains and losses resulting from foreign currency transactions, which are not material, are included in selling, general and administrative (“SG&A”) expenses in the consolidated statements of operations and comprehensive income. Cash and Cash Equivalents. The Company considers all highly liquid investments with maturities of three months or less when purchased 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 a combination of past experience and current trends. In estimating the allowance for expected credit losses, consideration is given to the current aging of receivables and a specific review for potential bad debts. The resulting bad debt expense is included in SG&A expenses in the consolidated statements of operations and comprehensive income. Receivables are written-off when deemed uncollectible. Leases. Effective January 1, 2019, operating leases are included in operating lease right of use assets and operating lease liabilities on the consolidated balance sheets, see Note 2 – Accounting Standards Update. 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. Business Combinations and Related Acquired Intangible Assets and Goodwill. The Company records all tangible and intangible assets acquired and liabilities assumed in a business combination at fair value as of the acquisition date in accordance with Accounting Standards Codification ("ASC") 805 Business Combinations. 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. The valuations are based on information that existed as of the acquisition date. During the measurement period, which shall 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. The excess amount of the aggregated purchase consideration paid over the fair value of the net of assets acquired and liabilities assumed is recorded as goodwill. Goodwill is evaluated for impairment annually or more frequently if an event occurs or circumstances change, such as material deterioration in performance that would indicate an impairment may exist. The Company performs the annual impairment assessment as of October 31^stfor each of its four 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, then the Company performs a quantitative assessment and calculates the estimated fair value of the reporting unit. If the carrying amount of the reporting unit exceeds the estimated fair value, an impairment charge would be 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 the significance of the excess of the reporting units’ estimated fair value over carrying amount at the last quantitative assessment date, the amount of time in between quantitative fair value assessments and the date of the acquisitions. 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. Principal factors used in the relief from royalty method that require judgment are projected net sales, discount rates, royalty rates and terminal growth assumptions. The estimated fair value of each trademark is compared to 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. 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, backlog and favorable contracts 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, non-compete agreements and in-use software are amortized using the straight-line method. 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 2019, 2018 and 2017. 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 and differences in estimates and actual payments for claims are recognized in the period that the estimates changed or the payments were 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. Stock-Based Compensation. The Company records compensation expense for restricted stock awards and restricted stock units based on the fair market value of the awards on the date of grant. The fair value of stock-based compensation awards less the estimated forfeitures is amortized over the vesting period of the award. Compensation expense for performance-based awards is measured based on the amount of shares ultimately expected to vest, estimated at each reporting date based on management’s expectations regarding the relevant performance criteria. The Company accounts for employee stock purchase plan shares based on an estimated fair market value using a Black-Scholes valuation model. This methodology requires the use of subjective assumptions such as expected stock price volatility. 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 Apex and Oxford segments 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 from the ECS segment are primarily from the U.S. government and are considered to have low credit risk.

Use of Estimates. The preparation of financial statements in conformity with generally accepted accounting principles 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. Actual results could differ from those estimates.

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. Revenues are recognized net of variable consideration to the extent it is probable a significant reversal of revenues will not occur in subsequent periods, see Note 11 – Revenues.

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, and as such, their 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 income (loss) as a separate component of stockholders’ equity. Gains and losses resulting from foreign currency transactions, which are not material, are included in selling, general and administrative (“SG&A”) expenses in the consolidated statements of operations and comprehensive income.

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 a combination of past experience and current trends. In estimating the allowance for expected credit losses, consideration is given to the current aging of receivables and a specific review for potential bad debts. The resulting bad debt expense is included in SG&A expenses in the consolidated statements of operations and comprehensive income. Receivables are written-off when deemed uncollectible.

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.

Business Combinations and Related Acquired Intangible Assets and Goodwill. The Company records all tangible and intangible assets acquired and liabilities assumed in a business combination at fair value as of the acquisition date in accordance with Accounting Standards Codification ("ASC") 805 Business Combinations. 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. The valuations are based on information that existed as of the acquisition date. During the measurement period, which shall 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

The excess amount of the aggregated purchase consideration paid over the fair value of the net of assets acquired and liabilities assumed is recorded as goodwill. Goodwill is evaluated for impairment annually or more frequently if an event occurs or circumstances change, such as material deterioration in performance that would indicate an impairment may exist. The Company performs the annual impairment assessment as of October 31^stfor each of its four 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, then the Company performs a quantitative assessment and calculates the estimated fair value of the reporting unit. If the carrying amount of the reporting unit exceeds the estimated fair value, an impairment charge would be 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 the significance of the excess of the reporting units’ estimated fair value over carrying amount at the last quantitative assessment date, the amount of time in between quantitative fair value assessments and the date of the acquisitions.

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. Principal factors used in the relief from royalty method that require judgment are projected net sales, discount rates, royalty rates and terminal growth assumptions. The estimated fair value of each trademark is compared to 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.

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, backlog and favorable contracts 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, non-compete agreements and in-use software are amortized using the straight-line method.

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 2019, 2018 and 2017.

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 and differences in estimates and actual payments for claims are recognized in the period that the estimates changed or the payments were 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.

Stock-Based Compensation. The Company records compensation expense for restricted stock awards and restricted stock units based on the fair market value of the awards on the date of grant. The fair value of stock-based compensation awards less the estimated forfeitures is amortized over the vesting period of the award. Compensation expense for performance-based awards is measured based on the amount of shares ultimately expected to vest, estimated at each reporting date based on management’s expectations regarding the relevant performance criteria. The Company accounts for employee stock purchase plan shares based on an estimated fair market value using a Black-Scholes valuation model. This methodology requires the use of subjective assumptions such as expected stock price volatility.

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 Apex and Oxford segments 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 from the ECS segment are primarily from the U.S. government and are considered to have low credit risk.