General (Notes)	6 Months Ended
	Jun. 30, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Financial Statement Presentation	Basis of presentation — The accompanying unaudited 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. Certain information and note disclosures normally included in annual financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to those rules and regulations. The December 31, 2020 balance sheet was derived from audited financial statements. The financial statements include adjustments consisting of normal recurring items, which, in the opinion of management, are necessary for a fair presentation of the financial position of ASGN Incorporated and its subsidiaries ("ASGN" or the "Company") and its results of operations for the interim dates and periods set forth herein. The results for any of the interim periods are not necessarily indicative of the results to be expected for the full year or any other period. This Quarterly Report on Form 10-Q should be read in conjunction with the Company’s Annual Report on Form 10-K for the year ended December 31, 2020 ("2020 10-K"). Retrospective reclassifications have been made to prior period financial statements and disclosures to present the Oxford Global Resources business unit ("Oxford business") as discontinued operations (see Note 2. Assets Held for Sale and Discontinued Operations). Unless otherwise noted, amounts and disclosures included herein relate to our continuing operations. Certain reclassifications were also made to prior year amounts to conform with current year presentation.