Income Taxes (Details) - USD ($)	12 Months Ended
Income Taxes (Details) - USD ($)	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Components of Deferred Tax Assets [Abstract]
Deferred Tax Assets, Net	$ 22,000,000	$ 34,000,000
Deferred Tax Assets, Valuation Allowance	(1,000,000)	(1,000,000)
Deferred Tax Assets, Net of Valuation Allowance, Total	56,000,000	68,000,000
Deferred Tax Assets, Gross [Abstract]
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits	11,000,000	12,000,000
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Self Insurance	4,000,000	4,000,000
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Deferred Rent	3,000,000	2,000,000
Deferred Tax Assets, Tax Deferred Expenses, Capitalized Software	0	13,000,000
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Share-Based Compensation Cost	15,000,000	15,000,000
DeferredTaxAssetsOperatingLeaseLiabilities	22,000,000	22,000,000
Deferred Tax Assets, Tax Deferred Expense, Other	2,000,000	1,000,000
Deferred Tax Assets, Gross, Total	57,000,000	69,000,000
Components of Deferred Tax Liabilities [Abstract]
Deferred Tax Liabilities, Deferred Expense	(4,000,000)	(4,000,000)
Deferred Tax Liabilities, Property, Plant and Equipment	(3,000,000)	(5,000,000)
Deferred Tax Liabilities, Deferred Expense, Capitalized Software	(2,000,000)	0
Deferred Tax Liabilities, Other	(3,000,000)	(3,000,000)
Deferred Tax Liabilities, Leasing Arrangements	(19,000,000)	(19,000,000)
Deferred Tax Liabilities, Goodwill and Intangible Assets	(3,000,000)	(3,000,000)
Deferred Tax Liabilities, Net	(34,000,000)	(34,000,000)
Unrecognized Tax Benefits, Period Increase (Decrease)	0	0	$ 0
Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)	(7,000,000)	41,000,000	49,000,000
Current State and Local Tax Expense (Benefit)	(2,000,000)	8,000,000	9,000,000
Current Income Tax Expense (Benefit), Total	(9,000,000)	49,000,000	58,000,000
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred Federal Income Tax Expense (Benefit)	10,000,000	(11,000,000)	(3,000,000)
Deferred State and Local Income Tax Expense (Benefit)	2,000,000	(3,000,000)	(1,000,000)
Deferred Federal, State and Local, Tax Expense (Benefit), Total	12,000,000	(14,000,000)	(4,000,000)
Income Tax Expense (Benefit), Total	3,000,000	35,000,000	54,000,000
Income Tax Expense (Benefit), Effective Income Tax Rate Reconciliation, Amount [Abstract]
Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount	$ (1,000,000)	$ 26,000,000	$ 47,000,000
Tax Jurisdiction of Domicile [Extensible Enumeration]	UNITED STATES	UNITED STATES	UNITED STATES
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent	$ 1,000,000	$ 4,000,000	$ 7,000,000
Effective Income Tax Rate Reconciliation, Tax Credit, Research, Amount	(3,000,000)	(1,000,000)	(1,000,000)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-Based Payment Arrangement, Amount	1,000,000	1,000,000	(4,000,000)
EffectiveIncomeTaxRateReconciliationNondeductibleExpenseOfficersCompensationAmount [Line Items]	2,000,000	3,000,000	2,000,000
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Other, Amount	2,000,000	1,000,000	1,000,000
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Meals and Entertainment, Amount	1,000,000	1,000,000	2,000,000
Income Tax Expense (Benefit), Total	$ 3,000,000	$ 35,000,000	$ 54,000,000
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%	21.00%	21.00%
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent	(14.00%)	3.00%	3.00%
Effective Income Tax Rate Reconciliation, Tax Credit, Research, Percent	70.00%	(1.00%)	(1.00%)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-Based Payment Arrangement, Percent	(33.00%)	1.00%	(2.00%)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Officer's Compensation, Percent	(42.00%)	2.00%	1.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Other, Percent	(40.00%)	1.00%	1.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Meals and Entertainment, Percent	(37.00%)	1.00%	1.00%
Effective Income Tax Rate Reconciliation, Percent	(75.00%)	28.00%	24.00%
Unrecognized Tax Benefits, Period Increase (Decrease)	$ 0	$ 0	$ 0
ExpensableMember [Member]
Deferred Tax Assets, Operating Loss Carryforwards, Domestic	$ 1,000,000

INSPERITY, INC. 10-K