Summary of Significant Accounting Policies	12 Months Ended
Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Summary of Significant Accounting Policie	Summary of Significant Accounting Policies Principles of Consolidation. The consolidated financial statements include the accounts of On Assignment, Inc. 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 from contract assignments, net of billing adjustments and discounts, are recognized when earned, based on hours worked by the Company’s contract professionals. Permanent placement revenues (direct hire and conversion fees) are recognized as revenues when employment candidates or contract professionals begin permanent employment. The Company records a sales allowance against consolidated revenues for estimated billing adjustments based on historical experience. The billing adjustment reserve includes an allowance for fallouts, which are permanent placement candidates that do not remain with the client through the contingency period, which is typically 90 days or less. When a fallout occurs the Company will generally find a replacement candidate at no additional cost to the client. The Company includes reimbursed expenses in revenues and the associated amounts of expenses in costs of services. The Company has direct contractual relationships with its customers, bears the risks and rewards of the transactions and has the discretion to select the contract professionals and establish the price of services provided, as such the Company is a principal and recognizes revenues on a gross basis. 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 assignment expenses, many of which are reimbursable by the client. 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 makes a comprehensive review of 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, Cash Equivalents and Restricted Cash. The Company considers all money market funds and highly liquid investments with maturities of three months or less when purchased to be cash equivalents, and from time to time it may include restricted cash related to the Company's deferred compensation plan. Accounts Receivable Allowances. The Company estimates an allowance for (i) expected credit losses (the inability of customers to make required payments), (ii) assignment revenue billing adjustments (e.g., bill rate adjustments, time card adjustments, early pay discounts) and (iii) fallouts (permanent placements that do not complete the contingency period, which is typically 90 days or less). 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 and assignment revenue billing adjustments and fallouts are reported as reductions to revenues in the consolidated statements of operations and comprehensive income. Receivables are written-off when deemed uncollectible. Leases. Office space leases range from six months to 11 years. Rent expense is recognized on a straight-line basis over the lease term. Differences between the straight-line rent expense and amounts payable under the leases are recorded as deferred rent which is included in other current liabilities and other long-term liabilities, as appropriate, in the consolidated balance sheets. This includes the impact of both scheduled rent increases and free or reduced periods. Allowances provided by landlords for leasehold improvements are included in deferred rent. 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. Assets acquired and liabilities assumed are measured at their estimated fair values as of the acquisition date. The Company determines the estimated fair values after review and consideration of relevant information including discounted cash flows and estimates made by management. Accordingly, these can be affected by future performance and other factors, which may cause final amounts to differ materially from original estimates. The preliminary estimates of the fair value of assets acquired and liabilities assumed are subject to change during a one-year measurement period. Adjustments to these preliminary estimates that are identified during the one-year measurement period are recognized in the reporting period in which the adjustments are determined. The excess of consideration transferred over the fair value of the net identifiable tangible and intangible assets acquired and liabilities assumed is recognized as goodwill. Goodwill acquired in business combinations is assigned to the reporting units expected to benefit from the combination as of the acquisition date. Acquisition-related costs are expensed as incurred. Goodwill and Identifiable Intangible Assets. Goodwill and indefinite-lived intangible assets (consisting entirely of trademarks) are tested for impairment on an annual basis as of October 31. Interim testing of goodwill and indefinite-lived intangible assets for impairment also occurs whenever an event occurs or circumstances change that would more likely than not reduce the fair value of a reporting unit or asset below its carrying value. 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 may first assess qualitative factors (Step 0) to determine whether it is necessary to perform a quantitative goodwill impairment test. If it is determined that a quantitative test is required (Step 1), the Company determines the fair value of each reporting unit by weighting fair value estimates using three accepted valuation methodologies: (i) an income approach, specifically a discounted cash flow analysis, (ii) a market approach, specifically the guideline company method and (iii) another market approach, specifically the similar transactions method. If after performing Step 1 of the goodwill impairment test, the fair value of equity of any reporting unit does not exceed the carrying value of equity, the Company performs a second step (“Step 2”) of the goodwill impairment test for that reporting unit. Step 2 measures the amount of goodwill impairment by comparing the implied fair value of the respective reporting unit's goodwill after estimating the fair value of specifically identifiable intangible assets, with the carrying value of that goodwill. The implied fair value of goodwill is determined using the same approach utilized to estimate the amount of goodwill recognized in a business combination. A qualitative analysis is performed for trademarks to determine if there were any indicators that the carrying value 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 value to determine if impairment exists. 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 relationships 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. There were no impairments of goodwill, trademarks or finite-lived intangible assets in 2017, 2016 and 2015. 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 value 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 2017, 2016 and 2015. 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, both per occurrence and in the aggregate. 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. 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 stock options granted and employee stock purchase plan shares based on an estimated fair market value using a Black-Scholes option valuation model. This methodology requires the use of subjective assumptions including expected stock price volatility and the estimated life of each award. The fair value of equity-based compensation awards less the estimated forfeitures is amortized over the vesting period of the award. Concentration of Credit Risk. Financial instruments that potentially subject the Company to credit risks consist primarily of cash, cash equivalents and restricted cash and trade receivables. The Company places its cash and cash equivalents in low risk investments with quality credit institutions and limits the amount of credit exposure with any single institution above FDIC insured limits. Concentration of credit risk with respect to accounts receivable is limited because of the large number of geographically dispersed customers, thus spreading the trade credit risk. The Company performs ongoing credit evaluations to identify risks and maintains an allowance to address these risks.

Summary of Significant Accounting Policies

Principles of Consolidation. The consolidated financial statements include the accounts of On Assignment, Inc. 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 from contract assignments, net of billing adjustments and discounts, are recognized when earned, based on hours worked by the Company’s contract professionals. Permanent placement revenues (direct hire and conversion fees) are recognized as revenues when employment candidates or contract professionals begin permanent employment. The Company records a sales allowance against consolidated revenues for estimated billing adjustments based on historical experience. The billing adjustment reserve includes an allowance for fallouts, which are permanent placement candidates that do not remain with the client through the contingency period, which is typically 90 days or less. When a fallout occurs the Company will generally find a replacement candidate at no additional cost to the client. The Company includes reimbursed expenses in revenues and the associated amounts of expenses in costs of services.

The Company has direct contractual relationships with its customers, bears the risks and rewards of the transactions and has the discretion to select the contract professionals and establish the price of services provided, as such the Company is a principal and recognizes revenues on a gross basis.

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 assignment expenses, many of which are reimbursable by the client.

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 makes a comprehensive review of 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, Cash Equivalents and Restricted Cash. The Company considers all money market funds and highly liquid investments with maturities of three months or less when purchased to be cash equivalents, and from time to time it may include restricted cash related to the Company's deferred compensation plan.

Accounts Receivable Allowances. The Company estimates an allowance for (i) expected credit losses (the inability of customers to make required payments), (ii) assignment revenue billing adjustments (e.g., bill rate adjustments, time card adjustments, early pay discounts) and (iii) fallouts (permanent placements that do not complete the contingency period, which is typically 90 days or less). 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 and assignment revenue billing adjustments and fallouts are reported as reductions to revenues in the consolidated statements of operations and comprehensive income. Receivables are written-off when deemed uncollectible.

Leases. Office space leases range from six months to 11 years. Rent expense is recognized on a straight-line basis over the lease term. Differences between the straight-line rent expense and amounts payable under the leases are recorded as deferred rent which is included in other current liabilities and other long-term liabilities, as appropriate, in the consolidated balance sheets. This includes the impact of both scheduled rent increases and free or reduced periods. Allowances provided by landlords for leasehold improvements are included in deferred rent.

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. Assets acquired and liabilities assumed are measured at their estimated fair values as of the acquisition date. The Company determines the estimated fair values after review and consideration of relevant information including discounted cash flows and estimates made by management. Accordingly, these can be affected by future performance and other factors, which may cause final amounts to differ materially from original estimates. The preliminary estimates of the fair value of assets acquired and liabilities assumed are subject to change during a one-year measurement period. Adjustments to these preliminary estimates that are identified during the one-year measurement period are recognized in the reporting period in which the adjustments are determined.

The excess of consideration transferred over the fair value of the net identifiable tangible and intangible assets acquired and liabilities assumed is recognized as goodwill. Goodwill acquired in business combinations is assigned to the reporting units expected to benefit from the combination as of the acquisition date. Acquisition-related costs are expensed as incurred.

Goodwill and Identifiable Intangible Assets. Goodwill and indefinite-lived intangible assets (consisting entirely of trademarks) are tested for impairment on an annual basis as of October 31. Interim testing of goodwill and indefinite-lived intangible assets for impairment also occurs whenever an event occurs or circumstances change that would more likely than not reduce the fair value of a reporting unit or asset below its carrying value.

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 may first assess qualitative factors (Step 0) to determine whether it is necessary to perform a quantitative goodwill impairment test. If it is determined that a quantitative test is required (Step 1), the Company determines the fair value of each reporting unit by weighting fair value estimates using three accepted valuation methodologies: (i) an income approach, specifically a discounted cash flow analysis, (ii) a market approach, specifically the guideline company method and (iii) another market approach, specifically the similar transactions method. If after performing Step 1 of the goodwill impairment test, the fair value of equity of any reporting unit does not exceed the carrying value of equity, the Company performs a second step (“Step 2”) of the goodwill impairment test for that reporting unit. Step 2 measures the amount of goodwill impairment by comparing the implied fair value of the respective reporting unit's goodwill after estimating the fair value of specifically identifiable intangible assets, with the carrying value of that goodwill. The implied fair value of goodwill is determined using the same approach utilized to estimate the amount of goodwill recognized in a business combination.

A qualitative analysis is performed for trademarks to determine if there were any indicators that the carrying value 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 value to determine if impairment exists.

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 relationships 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.

There were no impairments of goodwill, trademarks or finite-lived intangible assets in 2017, 2016 and 2015.

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 value 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 2017, 2016 and 2015.

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, both per occurrence and in the aggregate. 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. 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 stock options granted and employee stock purchase plan shares based on an estimated fair market value using a Black-Scholes option valuation model. This methodology requires the use of subjective assumptions including expected stock price volatility and the estimated life of each award. The fair value of equity-based compensation awards less the estimated forfeitures is amortized over the vesting period of the award.

Concentration of Credit Risk. Financial instruments that potentially subject the Company to credit risks consist primarily of cash, cash equivalents and restricted cash and trade receivables. The Company places its cash and cash equivalents in low risk investments with quality credit institutions and limits the amount of credit exposure with any single institution above FDIC insured limits. Concentration of credit risk with respect to accounts receivable is limited because of the large number of geographically dispersed customers, thus spreading the trade credit risk. The Company performs ongoing credit evaluations to identify risks and maintains an allowance to address these risks.