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Stockholders' Equity (Details) (USD $) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2014 | Jul. 21, 2014
Equity [Abstract] |  |
Stock Repurchased and Retired During Period, Value | $ 100,048,000 us-gaap_StockRepurchasedAndRetiredDuringPeriodValue |
Share repurchase program, amount authorized |  | 100,000,000 asgn_Sharerepurchaseprogramamountauthorized
Stock Repurchased and Retired During Period, Shares | 3,400,000 us-gaap_StockRepurchasedAndRetiredDuringPeriodShares |
Additional Paid-in Capital [Member] |  |
Equity [Abstract] |  |
Stock Repurchased and Retired During Period, Value | (31,194,000)us-gaap_StockRepurchasedAndRetiredDuringPeriodValue / us-gaap_StatementEquityComponentsAxis = us-gaap_AdditionalPaidInCapitalMember |
Retained Earnings [Member] |  |
Equity [Abstract] |  |
Stock Repurchased and Retired During Period, Value | $ (68,821,000)us-gaap_StockRepurchasedAndRetiredDuringPeriodValue / us-gaap_StatementEquityComponentsAxis = us-gaap_RetainedEarningsMember |

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- Definition Share repurchase program, amount authorized + References No definition available. + Details
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- Details
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- Definition Number of shares that have been repurchased and retired during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29, 30 -Article 5 + Details
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- Definition Equity impact of the value of stock that has been repurchased and retired during the period. The excess of the purchase price over par value can be charged against retained earnings (once the excess is fully allocated to additional paid in capital). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29, 30, 31 -Article 5 + Details
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