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Income Taxes - Additional Information (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Income Taxes Disclosure [Line Items]        
Effective Income Tax Rate Reconciliation, Percent 53.20%us-gaap_EffectiveIncomeTaxRateContinuingOperations 37.60%us-gaap_EffectiveIncomeTaxRateContinuingOperations 39.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations  
Income Tax Expense Related to Revaluation of Deferred Taxes $ 2,600,000us-gaap_IncomeTaxExpenseBenefitContinuingOperationsAdjustmentOfDeferredTaxAssetLiability $ 300,000us-gaap_IncomeTaxExpenseBenefitContinuingOperationsAdjustmentOfDeferredTaxAssetLiability $ 400,000us-gaap_IncomeTaxExpenseBenefitContinuingOperationsAdjustmentOfDeferredTaxAssetLiability  
Valuation allowance related to operating loss carryforwards 1,900,000us-gaap_OperatingLossCarryforwardsValuationAllowance 2,000,000us-gaap_OperatingLossCarryforwardsValuationAllowance    
Current tax expense (benefit) (26,263,000)us-gaap_CurrentIncomeTaxExpenseBenefit 14,617,000us-gaap_CurrentIncomeTaxExpenseBenefit 12,686,000us-gaap_CurrentIncomeTaxExpenseBenefit  
Valuation allowance related to tax credit carryforwards 5,000,000us-gaap_TaxCreditCarryforwardValuationAllowance      
Increase in valuation allowance amount 1,800,000us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount 3,700,000us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount 1,100,000us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount  
Employment tax credit 10,500,000us-gaap_IncomeTaxReconciliationTaxCredits      
Unrecognized tax benefits 2,471,000us-gaap_UnrecognizedTaxBenefits 2,300,000us-gaap_UnrecognizedTaxBenefits 2,710,000us-gaap_UnrecognizedTaxBenefits 2,516,000us-gaap_UnrecognizedTaxBenefits
Reasonably possible amount of unrecognized tax benefits to be settled within next year 2,600,000us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleAmountOfUnrecordedBenefit      
Unrecognized tax benefits, Interest 800,000us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued      
Unrecognized tax benefits, income tax penalties and interest accrued 800,000us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued 700,000us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued    
Income tax expense 17,579,000us-gaap_IncomeTaxExpenseBenefit 7,937,000us-gaap_IncomeTaxExpenseBenefit 20,344,000us-gaap_IncomeTaxExpenseBenefit  
Canadian Subsidiary        
Income Taxes Disclosure [Line Items]        
Undistributed earnings 1,900,000us-gaap_UndistributedEarnings
/ dei_LegalEntityAxis
= tnet_CanadianSubsidiaryMember		      
Uncertain Tax Positions        
Income Taxes Disclosure [Line Items]        
Unrecognized tax benefits 3,200,000us-gaap_UnrecognizedTaxBenefits
/ us-gaap_InformationByFinancialStatementLineItemAxis
= tnet_UncertainTaxPositionsMember		 2,900,000us-gaap_UnrecognizedTaxBenefits
/ us-gaap_InformationByFinancialStatementLineItemAxis
= tnet_UncertainTaxPositionsMember		    
Income tax expense 100,000us-gaap_IncomeTaxExpenseBenefit
/ us-gaap_InformationByFinancialStatementLineItemAxis
= tnet_UncertainTaxPositionsMember		 100,000us-gaap_IncomeTaxExpenseBenefit
/ us-gaap_InformationByFinancialStatementLineItemAxis
= tnet_UncertainTaxPositionsMember		 200,000us-gaap_IncomeTaxExpenseBenefit
/ us-gaap_InformationByFinancialStatementLineItemAxis
= tnet_UncertainTaxPositionsMember		  
Net Operating Loss Carryforwards        
Income Taxes Disclosure [Line Items]        
Current tax expense (benefit) 24,300,000us-gaap_CurrentIncomeTaxExpenseBenefit
/ us-gaap_TaxCreditCarryforwardAxis
= tnet_NetOperatingLossCarryforwardsMember		      
Stock Option Exercises And Net Operating Loss Carryforward        
Income Taxes Disclosure [Line Items]        
Current tax expense (benefit) 9,700,000us-gaap_CurrentIncomeTaxExpenseBenefit
/ us-gaap_TaxCreditCarryforwardAxis
= tnet_StockOptionExercisesAndNetOperatingLossCarryforwardMember		      
Federal        
Income Taxes Disclosure [Line Items]        
Net operating loss carryforwards 8,600,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember		      
Net operating loss carryforwards, expiration period 2030      
Operating loss carryforwards expire because of annual limitation 3,800,000tnet_OperatingLossCarryforwardsExpireBecauseOfAnnualLimitation
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember		      
Valuation allowance related to tax credit carryforwards 100,000us-gaap_TaxCreditCarryforwardValuationAllowance
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember		 100,000us-gaap_TaxCreditCarryforwardValuationAllowance
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember		    
Tax credit carryforwards 3,900,000us-gaap_TaxCreditCarryforwardAmount
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember		      
Tax credit carryforwards to be expire amount 1,800,000tnet_TaxCreditCarryforwardsToBeExpireAmount
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember		      
Tax credit carryforwards expiration period 2031      
Federal | Net Operating Loss Carryforwards        
Income Taxes Disclosure [Line Items]        
Stock option excess tax benefits 4,600,000us-gaap_DeferredTaxExpenseFromStockOptionsExercised
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember
/ us-gaap_TaxCreditCarryforwardAxis
= tnet_NetOperatingLossCarryforwardsMember		      
State        
Income Taxes Disclosure [Line Items]        
Net operating loss carryforwards 58,600,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember		      
Net operating loss carryforwards, expiration period 2015      
Operating loss carryforwards expire because of annual limitation 2,700,000tnet_OperatingLossCarryforwardsExpireBecauseOfAnnualLimitation
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember		      
Tax credit carryforwards 6,500,000us-gaap_TaxCreditCarryforwardAmount
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember		      
State | Net Operating Loss Carryforwards        
Income Taxes Disclosure [Line Items]        
Stock option excess tax benefits $ 19,300,000us-gaap_DeferredTaxExpenseFromStockOptionsExercised
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember
/ us-gaap_TaxCreditCarryforwardAxis
= tnet_NetOperatingLossCarryforwardsMember