Cash, Cash Equivalents and Investments - Cash, Cash Equivalents and Investments (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 228	$ 336
Investments	54	0
Restricted cash, cash equivalents and investments	942	1,280
Investments, noncurrent	135	0
Collateral for workers' compensation claims	187	162
Total	1,546	1,778
Insurance carriers security deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	15	15
Payroll funds collected
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	783	1,095
Collateral for health benefits claims
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	75	69
Collateral for workers' compensation claims
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	67	99
Collateral for workers' compensation claims	187	162
Collateral to secure standby letter of credit
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	2	2
Cash and cash equivalents
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	228	336
Investments	0	0
Restricted cash, cash equivalents and investments	939	1,277
Investments, noncurrent	0	0
Total	1,349	1,738
Cash and cash equivalents \| Insurance carriers security deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	15	15
Cash and cash equivalents \| Payroll funds collected
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	783	1,095
Cash and cash equivalents \| Collateral for health benefits claims
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	75	69
Cash and cash equivalents \| Collateral for workers' compensation claims
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	66	98
Collateral for workers' compensation claims	182	125
Cash and cash equivalents \| Collateral to secure standby letter of credit
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	0	0
Available for sale marketable securities
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	0	0
Investments	54	0
Restricted cash, cash equivalents and investments	1	1
Investments, noncurrent	135	0
Total	195	38
Available for sale marketable securities \| Insurance carriers security deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	0	0
Available for sale marketable securities \| Payroll funds collected
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	0	0
Available for sale marketable securities \| Collateral for health benefits claims
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	0	0
Available for sale marketable securities \| Collateral for workers' compensation claims
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	1	1
Collateral for workers' compensation claims	5	37
Available for sale marketable securities \| Collateral to secure standby letter of credit
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	0	0
Certificate of deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	0	0
Investments	0	0
Restricted cash, cash equivalents and investments	2	2
Investments, noncurrent	0	0
Total	2	2
Certificate of deposits \| Insurance carriers security deposits
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	0	0
Certificate of deposits \| Payroll funds collected
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	0	0
Certificate of deposits \| Collateral for health benefits claims
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	0	0
Certificate of deposits \| Collateral for workers' compensation claims
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	0	0
Collateral for workers' compensation claims	0	0
Certificate of deposits \| Collateral to secure standby letter of credit
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash, cash equivalents and investments	$ 2	$ 2

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), classified as current.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26626-111562

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in net income (available-for-sale), classified as noncurrent.

+ References

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecuritiesNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3044-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82887183&loc=d3e6676-107765

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.1)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_CashAndCashEquivalentsAtCarryingValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_RestrictedCashAndCashEquivalentsItemsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Cash and investments whose use in whole or in part is restricted for the long-term, generally by contractual agreements or regulatory requirements. For use in an unclassified balance sheet.

+ References

No definition available.

+ Details

Name:	us-gaap_RestrictedCashAndInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The current cash, cash equivalents and investments that are restricted as to withdrawal or usage. Restrictions may include legally restricted deposits held as compensating balances against short-term borrowing arrangements, contracts entered into with others, or entity statements of intention with regard to particular deposits; however, time deposits and short-term certificates of deposit are not generally included in legally restricted deposits. Excludes compensating balance arrangements that are not agreements which legally restrict the use of cash amounts shown on the balance sheet. Includes current cash equivalents and investments that are similarly restricted as to withdrawal, usage or disposal.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.1)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_RestrictedCashAndInvestmentsCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The noncurrent cash, cash equivalents and investments that is restricted as to withdrawal or usage. Restrictions may include legally restricted deposits held as compensating balances against borrowing arrangements, contracts entered into with others, or entity statements of intention with regard to particular deposits classified as long-term; that is not expected to be released from such existing restrictions within one year of the balance sheet date or operating cycle, whichever is longer. Excludes compensating balance arrangements that are not agreements which legally restrict the use of cash amounts shown on the balance sheet. Includes noncurrent cash equivalents and investments that are similarly restricted as to withdrawal, usage or disposal.

+ References

No definition available.

+ Details

Name:	us-gaap_RestrictedCashAndInvestmentsNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	tnet_CollateralbyTypeAxis=tnet_InsuranceCarriersSecurityDepositsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	tnet_CollateralbyTypeAxis=tnet_PayrollFundsCollectedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	tnet_CollateralbyTypeAxis=tnet_HealthBenefitClaimsCollateralMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	tnet_CollateralbyTypeAxis=tnet_WorkersCompensationClaimsCollateralMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	tnet_CollateralbyTypeAxis=tnet_StandbyLetterofCreditCollateralMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CashAndCashEquivalentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_AvailableforsaleSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CertificatesOfDepositMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: