Accrued Workers' Compensation Costs - Narrative (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Insurance [Abstract]
Collateral held by insurance carriers	$ 44	$ 45
Liability for unpaid claims and claims adjustment expense, collateral	$ 8	$ 8

Liability for Unpaid Claims and Claims Adjustment Expense, Collateral

No definition available.

Name:	tnet_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseCollateral
Namespace Prefix:	tnet_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant