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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) |  |  | $ 4.44us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit
Exercise price, high end of the range (in dollars per share) |  |  | $ 16.51us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit
Options outstanding (in shares) | 694,881us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber | 859,945us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber | 694,881us-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) | $ 11.33us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice | $ 10.61us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice | $ 11.33us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
Options exercisable - at the end of period (in shares) | 624,680us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber |  | 624,680us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber
Options exercisable - weighted average exercise price (in dollars per share) | $ 11.13us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice |  | $ 11.13us-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) |  |  | $ 4.44us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeOneMember
Exercise price, high end of the range (in dollars per share) |  |  | $ 8.26us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeOneMember
Options outstanding (in shares) | 141,321us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeOneMember |  | 141,321us-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) | $ 7.04us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeOneMember |  | $ 7.04us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeOneMember
Options exercisable - at the end of period (in shares) | 137,347us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeOneMember |  | 137,347us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeOneMember
Options exercisable - weighted average exercise price (in dollars per share) | $ 7.01us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeOneMember |  | $ 7.01us-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) |  |  | $ 9.19us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeTwoMember
Exercise price, high end of the range (in dollars per share) |  |  | $ 11.39us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeTwoMember
Options outstanding (in shares) | 231,836us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeTwoMember |  | 231,836us-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) | $ 11.00us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeTwoMember |  | $ 11.00us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeTwoMember
Options exercisable - at the end of period (in shares) | 198,891us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeTwoMember |  | 198,891us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeTwoMember
Options exercisable - weighted average exercise price (in dollars per share) | $ 11.09us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeTwoMember |  | $ 11.09us-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) |  |  | $ 11.75us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeThreeMember
Exercise price, high end of the range (in dollars per share) |  |  | $ 12.90us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeThreeMember
Options outstanding (in shares) | 211,724us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeThreeMember |  | 211,724us-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) | $ 12.40us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeThreeMember |  | $ 12.40us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeThreeMember
Options exercisable - at the end of period (in shares) | 211,724us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeThreeMember |  | 211,724us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeThreeMember
Options exercisable - weighted average exercise price (in dollars per share) | $ 12.40us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeThreeMember |  | $ 12.40us-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) |  |  | $ 13.31us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFourMember
Exercise price, high end of the range (in dollars per share) |  |  | $ 13.58us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFourMember
Options outstanding (in shares) | 35,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFourMember |  | 35,000us-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) | $ 13.32us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFourMember |  | $ 13.32us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFourMember
Options exercisable - at the end of period (in shares) | 34,531us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFourMember |  | 34,531us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFourMember
Options exercisable - weighted average exercise price (in dollars per share) | $ 13.32us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFourMember |  | $ 13.32us-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) |  |  | $ 16.51us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFiveMember
Exercise price, high end of the range (in dollars per share) |  |  | $ 16.51us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFiveMember
Options outstanding (in shares) | 75,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFiveMember |  | 75,000us-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) | $ 16.51us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFiveMember |  | $ 16.51us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFiveMember
Options exercisable - at the end of period (in shares) | 42,187us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFiveMember |  | 42,187us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFiveMember
Options exercisable - weighted average exercise price (in dollars per share) | $ 16.51us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = asgn_ExercisePriceRangeFiveMember |  | $ 16.51us-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) | 4,267us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_AwardTypeAxis = asgn_IncentiveStockOptionsMember | 69,416us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_AwardTypeAxis = asgn_IncentiveStockOptionsMember | 4,267us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_AwardTypeAxis = asgn_IncentiveStockOptionsMember
Options exercisable - at the end of period (in shares) | 4,267us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber / us-gaap_AwardTypeAxis = asgn_IncentiveStockOptionsMember |  | 4,267us-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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