STOCK BASED COMPENSATION - Schedule of Stock-Based Compensation Expense (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock based compensation expense	$ 17	$ 17	$ 50	$ 48
Total stock based compensation capitalized	0	1	1	2
Income Statement Location [Axis]: us-gaap:GeneralAndAdministrativeExpense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock based compensation expense	10	9	28	23
Income Statement Location [Axis]: us-gaap:ResearchAndDevelopmentExpense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock based compensation expense	2	1	4	5
Income Statement Location [Axis]: us-gaap:SellingAndMarketingExpense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock based compensation expense	1	3	7	8
Income Statement Location [Axis]: tnet:CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortizationAndOperatingInsuranceAndClaimsCostsProduction
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock based compensation expense	$ 4	$ 4	$ 11	$ 12

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.

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+ Details