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Stock-based Compensation: Incentive Award Plan and Employee Stock Purchase Plan Stock-based Compensation: Incentive Award Plan and Employee Stock Purchase Plan (Range of Prices) (Details) (USD $) | 0 Months Ended | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Exercise price, low end of the range (in dollars per share) |  |  | $ 4.44 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit
Exercise price, high end of the range (in dollars per share) |  |  | $ 16.51 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit
Options outstanding (in shares) | 694,881 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber | 859,945 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber | 694,881 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
Options outstanding - weighted average remaining contractual life (years) | 3 years 6 months | 4 years 9 months 18 days | 3 years 6 months
Options outstanding - weighted average exercise price (in dollars per share) | $ 11.33 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice | $ 10.61 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice | $ 11.33 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
Options exercisable - at the end of period (in shares) | 624,680 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber |  | 624,680 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber
Options exercisable - weighted average exercise price (in dollars per share) | $ 11.13 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice |  | $ 11.13 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice
$2.82 - 6.68 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Exercise price, low end of the range (in dollars per share) |  |  | $ 4.44 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeOneMember
Exercise price, high end of the range (in dollars per share) |  |  | $ 8.26 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeOneMember
Options outstanding (in shares) | 141,321 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeOneMember |  | 141,321 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeOneMember
Options outstanding - weighted average remaining contractual life (years) |  |  | 4 years 2 months 12 days
Options outstanding - weighted average exercise price (in dollars per share) | $ 7.04 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeOneMember |  | $ 7.04 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeOneMember
Options exercisable - at the end of period (in shares) | 137,347 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeOneMember |  | 137,347 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeOneMember
Options exercisable - weighted average exercise price (in dollars per share) | $ 7.01 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeOneMember |  | $ 7.01 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeOneMember
$6.86 - 10.20 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Exercise price, low end of the range (in dollars per share) |  |  | $ 9.19 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeTwoMember
Exercise price, high end of the range (in dollars per share) |  |  | $ 11.39 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeTwoMember
Options outstanding (in shares) | 231,836 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeTwoMember |  | 231,836 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeTwoMember
Options outstanding - weighted average remaining contractual life (years) |  |  | 3 years 2 months 12 days
Options outstanding - weighted average exercise price (in dollars per share) | $ 11.00 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeTwoMember |  | $ 11.00 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeTwoMember
Options exercisable - at the end of period (in shares) | 198,891 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeTwoMember |  | 198,891 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeTwoMember
Options exercisable - weighted average exercise price (in dollars per share) | $ 11.09 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeTwoMember |  | $ 11.09 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeTwoMember
$10.46 - 11.39 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Exercise price, low end of the range (in dollars per share) |  |  | $ 11.75 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeThreeMember
Exercise price, high end of the range (in dollars per share) |  |  | $ 12.90 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeThreeMember
Options outstanding (in shares) | 211,724 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeThreeMember |  | 211,724 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeThreeMember
Options outstanding - weighted average remaining contractual life (years) |  |  | 2 years 1 month 6 days
Options outstanding - weighted average exercise price (in dollars per share) | $ 12.40 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeThreeMember |  | $ 12.40 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeThreeMember
Options exercisable - at the end of period (in shares) | 211,724 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeThreeMember |  | 211,724 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeThreeMember
Options exercisable - weighted average exercise price (in dollars per share) | $ 12.40 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeThreeMember |  | $ 12.40 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeThreeMember
$11.56 - 12.90 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Exercise price, low end of the range (in dollars per share) |  |  | $ 13.31 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFourMember
Exercise price, high end of the range (in dollars per share) |  |  | $ 13.58 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFourMember
Options outstanding (in shares) | 35,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFourMember |  | 35,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFourMember
Options outstanding - weighted average remaining contractual life (years) |  |  | 2 years
Options outstanding - weighted average exercise price (in dollars per share) | $ 13.32 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFourMember |  | $ 13.32 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFourMember
Options exercisable - at the end of period (in shares) | 34,531 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFourMember |  | 34,531 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFourMember
Options exercisable - weighted average exercise price (in dollars per share) | $ 13.32 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFourMember |  | $ 13.32 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFourMember
$12.91 - 16.51 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Exercise price, low end of the range (in dollars per share) |  |  | $ 16.51 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFiveMember
Exercise price, high end of the range (in dollars per share) |  |  | $ 16.51 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFiveMember
Options outstanding (in shares) | 75,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFiveMember |  | 75,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFiveMember
Options outstanding - weighted average remaining contractual life (years) |  |  | 7 years 8 months 12 days
Options outstanding - weighted average exercise price (in dollars per share) | $ 16.51 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFiveMember |  | $ 16.51 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFiveMember
Options exercisable - at the end of period (in shares) | 42,187 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFiveMember |  | 42,187 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFiveMember
Options exercisable - weighted average exercise price (in dollars per share) | $ 16.51 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFiveMember |  | $ 16.51 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFiveMember
Incentive Stock Options [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Options outstanding (in shares) | 4,267 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_AwardTypeAxis = asgn_IncentiveStockOptionsMember | 69,416 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_AwardTypeAxis = asgn_IncentiveStockOptionsMember | 4,267 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_AwardTypeAxis = asgn_IncentiveStockOptionsMember
Options exercisable - at the end of period (in shares) | 4,267 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber / us-gaap_AwardTypeAxis = asgn_IncentiveStockOptionsMember |  | 4,267 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber / us-gaap_AwardTypeAxis = asgn_IncentiveStockOptionsMember

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- Definition The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The floor of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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