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Income Taxes (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Deferred tax liabilities:      
Prepaid assets $ (9,291,000)us-gaap_DeferredTaxLiabilitiesDeferredExpense $ (5,084,000)us-gaap_DeferredTaxLiabilitiesDeferredExpense  
Depreciation (8,083,000)us-gaap_DeferredTaxLiabilitiesPropertyPlantAndEquipment (8,937,000)us-gaap_DeferredTaxLiabilitiesPropertyPlantAndEquipment  
Software development costs (2,252,000)us-gaap_DeferredTaxLiabilitiesDeferredExpenseCapitalizedSoftware (2,739,000)us-gaap_DeferredTaxLiabilitiesDeferredExpenseCapitalizedSoftware  
Total deferred tax liabilities (19,626,000)us-gaap_DeferredTaxLiabilities (16,760,000)us-gaap_DeferredTaxLiabilities  
Deferred tax assets:      
Accrued incentive compensation 7,204,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefits 3,909,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefits  
Net operating loss carryforward 1,556,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwards 1,773,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwards  
Workers’ compensation accruals 6,308,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsSelfInsurance 5,568,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsSelfInsurance  
Accrued rent 1,058,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsAccruedLiabilities 1,008,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsAccruedLiabilities  
Stock-based compensation 3,615,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost 3,742,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost  
Intangibles 1,575,000us-gaap_DeferredTaxAssetsGoodwillAndIntangibleAssets 837,000us-gaap_DeferredTaxAssetsGoodwillAndIntangibleAssets  
Minority investment impairment 1,003,000us-gaap_DeferredTaxAssetsInvestments 991,000us-gaap_DeferredTaxAssetsInvestments  
Other 551,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseOther 412,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseOther  
Total deferred tax assets 22,870,000us-gaap_DeferredTaxAssetsGross 18,240,000us-gaap_DeferredTaxAssetsGross  
Valuation allowance (1,003,000)us-gaap_DeferredTaxAssetsValuationAllowance (991,000)us-gaap_DeferredTaxAssetsValuationAllowance  
Total net deferred tax assets 21,867,000us-gaap_DeferredTaxAssetsNet 17,249,000us-gaap_DeferredTaxAssetsNet  
Net deferred tax assets 2,241,000us-gaap_DeferredTaxAssetsLiabilitiesNet 489,000us-gaap_DeferredTaxAssetsLiabilitiesNet  
Net current deferred tax assets 6,316,000us-gaap_DeferredTaxAssetsLiabilitiesNetCurrent 8,185,000us-gaap_DeferredTaxAssetsLiabilitiesNetCurrent  
Net noncurrent deferred tax liabilities (4,075,000)us-gaap_DeferredTaxAssetsLiabilitiesNetNoncurrent (7,696,000)us-gaap_DeferredTaxAssetsLiabilitiesNetNoncurrent  
Current income tax expense:      
Federal 18,034,000us-gaap_CurrentFederalTaxExpenseBenefit 20,476,000us-gaap_CurrentFederalTaxExpenseBenefit 29,280,000us-gaap_CurrentFederalTaxExpenseBenefit
State 3,322,000us-gaap_CurrentStateAndLocalTaxExpenseBenefit 3,512,000us-gaap_CurrentStateAndLocalTaxExpenseBenefit 4,351,000us-gaap_CurrentStateAndLocalTaxExpenseBenefit
Total current income tax expense 21,356,000us-gaap_CurrentIncomeTaxExpenseBenefit 23,988,000us-gaap_CurrentIncomeTaxExpenseBenefit 33,631,000us-gaap_CurrentIncomeTaxExpenseBenefit
Deferred income tax (benefit) expense:      
Federal (1,764,000)us-gaap_DeferredFederalIncomeTaxExpenseBenefit (2,258,000)us-gaap_DeferredFederalIncomeTaxExpenseBenefit (5,363,000)us-gaap_DeferredFederalIncomeTaxExpenseBenefit
State 31,000us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit (30,000)us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit (380,000)us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit
Total deferred income tax benefit (1,733,000)us-gaap_DeferredIncomeTaxExpenseBenefit (2,288,000)us-gaap_DeferredIncomeTaxExpenseBenefit (5,743,000)us-gaap_DeferredIncomeTaxExpenseBenefit
Total income tax expense 19,623,000us-gaap_IncomeTaxExpenseBenefit 21,700,000us-gaap_IncomeTaxExpenseBenefit 27,888,000us-gaap_IncomeTaxExpenseBenefit
Net income tax expense (benefit) on nonqualified stock option exercises 488,000us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation 1,259,000us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation 1,751,000us-gaap_AdjustmentsToAdditionalPaidInCapitalTaxEffectFromShareBasedCompensation
Reconciliation of income tax expense [Abstract]      
Assumed income tax rate (in hundredths) 35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate    
Expected income tax expense at 35% 16,670,000us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate 18,806,000us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate 23,901,000us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate
State income taxes, net of federal benefit 2,204,000us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes 2,286,000us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes 2,497,000us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes
Nondeductible expenses 1,939,000us-gaap_IncomeTaxReconciliationNondeductibleExpense 1,993,000us-gaap_IncomeTaxReconciliationNondeductibleExpense 1,663,000us-gaap_IncomeTaxReconciliationNondeductibleExpense
Section 199 benefits (592,000)us-gaap_IncomeTaxReconciliationDeductionsQualifiedProductionActivities (2,531,000)us-gaap_IncomeTaxReconciliationDeductionsQualifiedProductionActivities 0us-gaap_IncomeTaxReconciliationDeductionsQualifiedProductionActivities
Expense Management non-cash impairment 0us-gaap_IncomeTaxReconciliationNondeductibleExpenseImpairmentLosses 797,000us-gaap_IncomeTaxReconciliationNondeductibleExpenseImpairmentLosses 0us-gaap_IncomeTaxReconciliationNondeductibleExpenseImpairmentLosses
Valuation allowance related to TRE impairment 0us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance 938,000us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance 0us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance
Research and development credit (455,000)us-gaap_IncomeTaxReconciliationTaxCreditsResearch (534,000)us-gaap_IncomeTaxReconciliationTaxCreditsResearch 0us-gaap_IncomeTaxReconciliationTaxCreditsResearch
Other, net (143,000)us-gaap_IncomeTaxReconciliationOtherAdjustments (55,000)us-gaap_IncomeTaxReconciliationOtherAdjustments (173,000)us-gaap_IncomeTaxReconciliationOtherAdjustments
Total income tax expense 19,623,000us-gaap_IncomeTaxExpenseBenefit 21,700,000us-gaap_IncomeTaxExpenseBenefit 27,888,000us-gaap_IncomeTaxExpenseBenefit
Business Acquisition [Line Items]      
Section 199 tax benefits for the tax year 2013   530,000nsp_Section199TaxBenefitsfortheTaxYear2013  
Section 199 Tax Benefits for Tax Years 2009 to 2012   2,000,000nsp_Section199TaxBenefitsforTaxYears2009to2012  
Liability for Uncertain Tax Positions, Current 0us-gaap_LiabilityForUncertainTaxPositionsCurrent 0us-gaap_LiabilityForUncertainTaxPositionsCurrent 0us-gaap_LiabilityForUncertainTaxPositionsCurrent
ExpensAble [Member]      
Business Acquisition [Line Items]      
Operating Loss Carryforwards $ 4,200,000us-gaap_OperatingLossCarryforwards
/ us-gaap_BusinessAcquisitionAxis
= nsp_ExpensableMember		    
Minimum [Member] | ExpensAble [Member]      
Business Acquisition [Line Items]      
Net operating loss carryforwards, expiration date Jan. 01, 2023    
Maximum [Member] | ExpensAble [Member]      
Business Acquisition [Line Items]      
Net operating loss carryforwards, expiration date Jan. 01, 2030