Accrued Workers' Compensation Costs - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Insurance [Abstract]
Collateral held by insurance carriers	$ 46	$ 57
Collateral paid to carriers and offset against loss reserves	$ 9	$ 13

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