Workers' Compensation Loss Reserves - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Insurance [Abstract]
Collateral held by insurance carriers	$ 63	$ 66
Collateral paid to carriers and offset against loss reserves	$ (23)	$ (32)

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