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Workers' Compensation Loss Reserves - Additional Information (Details) - USD ($) $ in Millions | Sep. 30, 2018 | Dec. 31, 2017
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Insurance [Abstract] |  |
Collateral held by insurance carriers | $ 63 | $ 63
Collateral paid to carriers and offset against loss reserves | $ 18 | $ 23

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- Definition Collateral Held by Insurance Carriers + References No definition available. + Details
Name: | tnet_CollateralHeldByInsuranceCarriers
Namespace Prefix: | tnet_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Liability for Unpaid Claims and Claims Adjustment Expense, Collateral + References No definition available. + Details
Name: | tnet_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseCollateral
Namespace Prefix: | tnet_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_InsuranceAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration