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Unaudited Quarterly Financial Results (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified | 3 Months Ended | 12 Months Ended
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Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract] |  |  |  |  |  |  |  |  |  |  |
Revenues | $ 441,023 us-gaap_SalesRevenueServicesNet | $ 442,443 us-gaap_SalesRevenueServicesNet | $ 434,424 us-gaap_SalesRevenueServicesNet | $ 406,851 us-gaap_SalesRevenueServicesNet | $ 396,070 us-gaap_SalesRevenueServicesNet | $ 394,400 us-gaap_SalesRevenueServicesNet | $ 380,779 us-gaap_SalesRevenueServicesNet | $ 351,852 us-gaap_SalesRevenueServicesNet | $ 1,724,741 us-gaap_SalesRevenueServicesNet | $ 1,523,101 us-gaap_SalesRevenueServicesNet | $ 1,031,935 us-gaap_SalesRevenueServicesNet
Gross profit | 142,537 us-gaap_GrossProfit | 144,838 us-gaap_GrossProfit | 141,905 us-gaap_GrossProfit | 128,155 us-gaap_GrossProfit | 120,985 us-gaap_GrossProfit | 119,016 us-gaap_GrossProfit | 113,105 us-gaap_GrossProfit | 101,769 us-gaap_GrossProfit | 557,435 us-gaap_GrossProfit | 454,875 us-gaap_GrossProfit | 321,254 us-gaap_GrossProfit
Income from continuing operations | 19,526 us-gaap_IncomeLossFromContinuingOperations | 20,723 us-gaap_IncomeLossFromContinuingOperations | 19,641 us-gaap_IncomeLossFromContinuingOperations | 13,605 us-gaap_IncomeLossFromContinuingOperations | 16,321 us-gaap_IncomeLossFromContinuingOperations | 18,656 us-gaap_IncomeLossFromContinuingOperations | 6,649 us-gaap_IncomeLossFromContinuingOperations | 9,514 us-gaap_IncomeLossFromContinuingOperations | 73,495 us-gaap_IncomeLossFromContinuingOperations | 51,140 us-gaap_IncomeLossFromContinuingOperations | 33,463 us-gaap_IncomeLossFromContinuingOperations
Income from discontinued operations, net of income taxes | 947 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity | 1,282 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity | 1,148 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity | 312 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity | 16,089 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity | 1,494 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity | 690 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity | 15,099 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity | 3,689 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity | 2,532 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity | 9,190 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity
Net income | 20,473 us-gaap_NetIncomeLoss | 22,005 us-gaap_NetIncomeLoss | 20,789 us-gaap_NetIncomeLoss | 13,917 us-gaap_NetIncomeLoss | 32,410 us-gaap_NetIncomeLoss | 20,150 us-gaap_NetIncomeLoss | 7,339 us-gaap_NetIncomeLoss | 24,613 us-gaap_NetIncomeLoss | 77,184 us-gaap_NetIncomeLoss | 84,512 us-gaap_NetIncomeLoss | 42,653 us-gaap_NetIncomeLoss
Earnings Per Share, Basic [Abstract] |  |  |  |  |  |  |  |  |  |  |
Continuing operations | $ 0.38 us-gaap_IncomeLossFromContinuingOperationsPerBasicShare | $ 0.39 us-gaap_IncomeLossFromContinuingOperationsPerBasicShare | $ 0.36 us-gaap_IncomeLossFromContinuingOperationsPerBasicShare | $ 0.25 us-gaap_IncomeLossFromContinuingOperationsPerBasicShare | $ 0.30 us-gaap_IncomeLossFromContinuingOperationsPerBasicShare | $ 0.35 us-gaap_IncomeLossFromContinuingOperationsPerBasicShare | $ 0.12 us-gaap_IncomeLossFromContinuingOperationsPerBasicShare | $ 0.18 us-gaap_IncomeLossFromContinuingOperationsPerBasicShare | $ 1.38 us-gaap_IncomeLossFromContinuingOperationsPerBasicShare | $ 0.96 us-gaap_IncomeLossFromContinuingOperationsPerBasicShare | $ 0.72 us-gaap_IncomeLossFromContinuingOperationsPerBasicShare
Discontinued operations | $ 0.01 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare | $ 0.02 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare | $ 0.02 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare | $ 0.01 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare | $ 0.30 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare | $ 0.03 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare | $ 0.02 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare | $ 0.28 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare | $ 0.06 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare | $ 0.62 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare | $ 0.19 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare
Net income | $ 0.39 us-gaap_EarningsPerShareBasic | $ 0.41 us-gaap_EarningsPerShareBasic | $ 0.38 us-gaap_EarningsPerShareBasic | $ 0.26 us-gaap_EarningsPerShareBasic | $ 0.60 us-gaap_EarningsPerShareBasic | $ 0.38 us-gaap_EarningsPerShareBasic | $ 0.14 us-gaap_EarningsPerShareBasic | $ 0.46 us-gaap_EarningsPerShareBasic | $ 1.44 us-gaap_EarningsPerShareBasic | $ 1.58 us-gaap_EarningsPerShareBasic | $ 0.91 us-gaap_EarningsPerShareBasic
Earnings Per Share, Diluted [Abstract] |  |  |  |  |  |  |  |  |  |  |
Continuing operations | $ 0.37 us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare | $ 0.38 us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare | $ 0.36 us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare | $ 0.25 us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare | $ 0.30 us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare | $ 0.34 us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare | $ 0.12 us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare | $ 0.18 us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare | $ 1.35 us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare | $ 0.94 us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare | $ 0.70 us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare
Discontinued operations | $ 0.02 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare | $ 0.03 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare | $ 0.02 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare | $ 0.00 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare | $ 0.29 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare | $ 0.03 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare | $ 0.02 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare | $ 0.28 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare | $ 0.07 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare | $ 0.61 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare | $ 0.19 us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare
Net income | $ 0.39 us-gaap_EarningsPerShareDiluted | $ 0.41 us-gaap_EarningsPerShareDiluted | $ 0.38 us-gaap_EarningsPerShareDiluted | $ 0.25 us-gaap_EarningsPerShareDiluted | $ 0.59 us-gaap_EarningsPerShareDiluted | $ 0.37 us-gaap_EarningsPerShareDiluted | $ 0.14 us-gaap_EarningsPerShareDiluted | $ 0.46 us-gaap_EarningsPerShareDiluted | $ 1.42 us-gaap_EarningsPerShareDiluted | $ 1.55 us-gaap_EarningsPerShareDiluted | $ 0.89 us-gaap_EarningsPerShareDiluted
Apex Segment [Member] |  |  |  |  |  |  |  |  |  |  |
Quarterly Financial Information Disclosure [Abstract] |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 1,190,052 us-gaap_SalesRevenueServicesNet / us-gaap_StatementBusinessSegmentsAxis = asgn_ApexSegmentMember | 1,059,993 us-gaap_SalesRevenueServicesNet / us-gaap_StatementBusinessSegmentsAxis = asgn_ApexSegmentMember | 617,844 us-gaap_SalesRevenueServicesNet / us-gaap_StatementBusinessSegmentsAxis = asgn_ApexSegmentMember
Gross profit |  |  |  |  |  |  |  |  | $ 335,322 us-gaap_GrossProfit / us-gaap_StatementBusinessSegmentsAxis = asgn_ApexSegmentMember | $ 294,611 us-gaap_GrossProfit / us-gaap_StatementBusinessSegmentsAxis = asgn_ApexSegmentMember | $ 174,968 us-gaap_GrossProfit / us-gaap_StatementBusinessSegmentsAxis = asgn_ApexSegmentMember

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