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Goodwill and Identifiable Assets (Goodwill) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Goodwill [Roll Forward]
Gross goodwill	$ 510,164us-gaap_GoodwillGross	$ 512,060us-gaap_GoodwillGross	$ 514,426us-gaap_GoodwillGross
Accumulated impairment loss	0us-gaap_GoodwillImpairedAccumulatedImpairmentLoss	0us-gaap_GoodwillImpairedAccumulatedImpairmentLoss	0us-gaap_GoodwillImpairedAccumulatedImpairmentLoss
Goodwill	510,164us-gaap_Goodwill	512,060us-gaap_Goodwill	514,426us-gaap_Goodwill
Translation adjustment	(1,896)us-gaap_GoodwillTranslationAdjustments	(2,366)us-gaap_GoodwillTranslationAdjustments
Apex
Goodwill [Roll Forward]
Gross goodwill	286,539us-gaap_GoodwillGross / us-gaap_StatementBusinessSegmentsAxis = asgn_ApexSegmentMember	287,951us-gaap_GoodwillGross / us-gaap_StatementBusinessSegmentsAxis = asgn_ApexSegmentMember	289,712us-gaap_GoodwillGross / us-gaap_StatementBusinessSegmentsAxis = asgn_ApexSegmentMember
Accumulated impairment loss	0us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = asgn_ApexSegmentMember	0us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = asgn_ApexSegmentMember	0us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = asgn_ApexSegmentMember
Goodwill	286,539us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = asgn_ApexSegmentMember	287,951us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = asgn_ApexSegmentMember	289,712us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = asgn_ApexSegmentMember
Translation adjustment	(1,412)us-gaap_GoodwillTranslationAdjustments / us-gaap_StatementBusinessSegmentsAxis = asgn_ApexSegmentMember	(1,761)us-gaap_GoodwillTranslationAdjustments / us-gaap_StatementBusinessSegmentsAxis = asgn_ApexSegmentMember
Oxford
Goodwill [Roll Forward]
Gross goodwill	219,934us-gaap_GoodwillGross / us-gaap_StatementBusinessSegmentsAxis = asgn_OxfordSegmentMember	220,030us-gaap_GoodwillGross / us-gaap_StatementBusinessSegmentsAxis = asgn_OxfordSegmentMember	220,150us-gaap_GoodwillGross / us-gaap_StatementBusinessSegmentsAxis = asgn_OxfordSegmentMember
Accumulated impairment loss	0us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = asgn_OxfordSegmentMember	0us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = asgn_OxfordSegmentMember	0us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = asgn_OxfordSegmentMember
Goodwill	219,934us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = asgn_OxfordSegmentMember	220,030us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = asgn_OxfordSegmentMember	220,150us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = asgn_OxfordSegmentMember
Translation adjustment	(96)us-gaap_GoodwillTranslationAdjustments / us-gaap_StatementBusinessSegmentsAxis = asgn_OxfordSegmentMember	(120)us-gaap_GoodwillTranslationAdjustments / us-gaap_StatementBusinessSegmentsAxis = asgn_OxfordSegmentMember
Life Sciences Europe
Goodwill [Roll Forward]
Gross goodwill	3,691us-gaap_GoodwillGross / us-gaap_StatementBusinessSegmentsAxis = asgn_LifeSciencesEuropeMember	4,079us-gaap_GoodwillGross / us-gaap_StatementBusinessSegmentsAxis = asgn_LifeSciencesEuropeMember	4,564us-gaap_GoodwillGross / us-gaap_StatementBusinessSegmentsAxis = asgn_LifeSciencesEuropeMember
Accumulated impairment loss	0us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = asgn_LifeSciencesEuropeMember	0us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = asgn_LifeSciencesEuropeMember	0us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / us-gaap_StatementBusinessSegmentsAxis = asgn_LifeSciencesEuropeMember
Goodwill	3,691us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = asgn_LifeSciencesEuropeMember	4,079us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = asgn_LifeSciencesEuropeMember	4,564us-gaap_Goodwill / us-gaap_StatementBusinessSegmentsAxis = asgn_LifeSciencesEuropeMember
Translation adjustment	$ (388)us-gaap_GoodwillTranslationAdjustments / us-gaap_StatementBusinessSegmentsAxis = asgn_LifeSciencesEuropeMember	$ (485)us-gaap_GoodwillTranslationAdjustments / us-gaap_StatementBusinessSegmentsAxis = asgn_LifeSciencesEuropeMember

X

- Definition

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267



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- Definition

Amount before accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

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-Section 50

-Paragraph 1

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Amount of accumulated impairment loss for an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 50

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Amount of increase (decrease) from foreign currency translation adjustments of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

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-Section 50

-Paragraph 1

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