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Accounting Policies (Details) (USD $) | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Oct. 01, 2010 | Sep. 30, 2010
Description of Business [Abstract] |  |  |  |  |
Number of years transition to comprehensive business performance solutions provider | 28 years |  |  |  |
Segment Reporting [Abstract] |  |  |  |  |
Number of reportable segments | 1 us-gaap_NumberOfReportableSegments |  |  |  |
Property, Plant and Equipment [Abstract] |  |  |  |  |
Amortization of capitalized computer software costs | $ 4,100,000 us-gaap_CapitalizedComputerSoftwareAmortization | $ 3,600,000 us-gaap_CapitalizedComputerSoftwareAmortization | $ 2,800,000 us-gaap_CapitalizedComputerSoftwareAmortization |  |
Unamortized computer software costs | 6,000,000 us-gaap_CapitalizedComputerSoftwareNet | 7,200,000 us-gaap_CapitalizedComputerSoftwareNet |  |  |
Property and Equipment, Estimated Useful Lives [Abstract] |  |  |  |  |
Asset Impairment Charges | 1,200,000 us-gaap_TangibleAssetImpairmentCharges |  |  |  |
Health Insurance Costs [Abstract] |  |  |  |  |
Number of days cash in advance of beginning of reporting quarter United establishes cash funding rates | 90 days |  |  |  |
Prepaid health insurance | 9,000,000 nsp_PrepaidHealthInsurance | 9,000,000 nsp_PrepaidHealthInsurance |  |  |
Required deposit for health care costs | 3,500,000 nsp_RequiredDepositForHealthCareCosts |  |  |  |
Amount which Plan Costs were less than the net premiums paid and owed | 23,500,000 nsp_PrepaidHealthInsuranceCurrentAndNoncurrent |  |  |  |
Prepaid health insurance, current | 14,500,000 nsp_PrepaidHealthInsuranceCurrent |  |  |  |
Premiums owed to United | 15,000,000 nsp_PremiumsOwedToUnited |  |  |  |
Workers' Compensation Costs [Abstract] |  |  |  |  |
Company's maximum economic burden for the first layer of claims per occurrence |  |  |  |  | 1,000,000 nsp_WorkersCompensationMaximumEconomicBurdenFirstLayerOfClaimsPerOccurrence
Company's maximum aggregate economic burden for claims in excess of $1 million per policy year |  |  |  | 5,000,000 nsp_WorkersCompensationMaximumAggregateEconomicBurdenForClaimsInExcessOfFirstLayerCapPerPolicyYear |
Reduction in accrued workers' compensation costs for changes in estimated losses | 2,900,000 nsp_DecreaseIncreaseInAccruedWorkersCompensationCostsForChangesInEstimatedLosses | 9,300,000 nsp_DecreaseIncreaseInAccruedWorkersCompensationCostsForChangesInEstimatedLosses |  |  |
U.S. Treasury rates that correspond with the weighted average estimated claim payout period (in hundredths) | 1.00%nsp_USTreasuryRatesThatCorrespondWithWeightedAverageEstimatedClaimPayoutPeriod | 0.80%nsp_USTreasuryRatesThatCorrespondWithWeightedAverageEstimatedClaimPayoutPeriod |  |  |
Incurred but not paid workers' compensation liabilities |  |  |  |  |
Beginning balance | 120,833,000 us-gaap_WorkersCompensationLiabilityCurrentAndNoncurrent | 111,685,000 us-gaap_WorkersCompensationLiabilityCurrentAndNoncurrent |  |  |
Accrued claims | 55,971,000 nsp_WorkersCompensationExpense | 42,900,000 nsp_WorkersCompensationExpense |  |  |
Present value discount | (1,998,000)nsp_WorkersCompensationDiscountChangedDuringPeriod | (1,169,000)nsp_WorkersCompensationDiscountChangedDuringPeriod |  |  |
Paid claims | (38,718,000)nsp_WorkersCompensationClaimsPaid | (32,583,000)nsp_WorkersCompensationClaimsPaid |  |  |
Ending balance | 136,088,000 us-gaap_WorkersCompensationLiabilityCurrentAndNoncurrent | 120,833,000 us-gaap_WorkersCompensationLiabilityCurrentAndNoncurrent | 111,685,000 us-gaap_WorkersCompensationLiabilityCurrentAndNoncurrent |  |
Current portion of accrued claims | 44,040,000 nsp_WorkersCompensationLiabilityCurrentNet | 51,928,000 nsp_WorkersCompensationLiabilityCurrentNet |  |  |
Long-term portion of accrued claims | 92,048,000 us-gaap_WorkersCompensationLiabilityNoncurrent | 68,905,000 us-gaap_WorkersCompensationLiabilityNoncurrent |  |  |
Ending Balance | 136,088,000 us-gaap_WorkersCompensationLiabilityCurrentAndNoncurrent | 120,833,000 us-gaap_WorkersCompensationLiabilityCurrentAndNoncurrent | 111,685,000 us-gaap_WorkersCompensationLiabilityCurrentAndNoncurrent |  |
Current portion of workers' compensation administrative fees accrued | 1,600,000 nsp_WorkersCompensationAdministrativeFeesAccrued | 1,000,000 nsp_WorkersCompensationAdministrativeFeesAccrued |  |  |
Undiscounted accrued workers' compensation costs | 145,800,000 nsp_UndiscountedAccruedWorkersCompensationCosts | 131,200,000 nsp_UndiscountedAccruedWorkersCompensationCosts |  |  |
Time period incurred claims expected to be paid recorded as restricted cash | 1 year |  |  |  |
Time period incurred claims expected to be paid, included in deposits, a long-term asset | Greater than 1 year |  |  |  |
Additional claim funds paid related to ACE Program | 7,200,000 nsp_AdditionalclaimfundspaidrelatedtoACEProgram |  |  |  |
Restricted Cash and Cash Equivalents, Current | 44,040,000 us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue | 51,928,000 us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue |  |  |
Deposits - workers' compensation | 113,934,000 nsp_DepositsWorkersCompensation | 81,878,000 nsp_DepositsWorkersCompensation |  |  |
Time period estimate of incurred claim costs to be paid included in short term liabilities | 1 year |  |  |  |
Time period incurred claims expected to be paid, included in long-term liabilities | Greater than 1 year |  |  |  |
Stock-Based Compensation [Abstract] |  |  |  |  |
Number of stock-based employee compensation plans | 2 nsp_NumberOfStockBasedEmployeeCompensationPlans |  |  |  |
Vesting period for Initial grants to new members of the Board of Directors | 3 years |  |  |  |
Vesting period for officers and other management to be eligible for restricted stock grants, minimum | 3 years |  |  |  |
Vesting period for officers and other management to be eligible for restricted stock grants, maximum | 5 years |  |  |  |
Percentage of annual grants issued to directors that are vested (in hundredths) | 100.00%nsp_Percentageofgrantsissuedtodirectorsthatarevestedforannualawardgrants. |  |  |  |
Worksite Employees [Member] |  |  |  |  |
Company-Sponsored 401 (k) Plans [Abstract] |  |  |  |  |
Percentage of eligible compensation matched, minimum (in hundredths) | 0.00%nsp_PercentageOfEligibleCompensationMatchedMinimum / us-gaap_TitleOfIndividualAxis = nsp_WorksiteEmployeesMember |  |  |  |
Percentage of eligible compensation matched, maximum (in hundredths) | 6.00%nsp_Percentageofeligiblecompensationmatchedmaximum / us-gaap_TitleOfIndividualAxis = nsp_WorksiteEmployeesMember |  |  |  |
Matching contributions to the Plans | 78,400,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_TitleOfIndividualAxis = nsp_WorksiteEmployeesMember | 71,700,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_TitleOfIndividualAxis = nsp_WorksiteEmployeesMember | 63,300,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_TitleOfIndividualAxis = nsp_WorksiteEmployeesMember |  |
Corporate Employees [Member] |  |  |  |  |
Company-Sponsored 401 (k) Plans [Abstract] |  |  |  |  |
Percentage match of eligible corporate employees' contributions (in hundredths) | 50.00%nsp_PercentageEntityMatchesOfEligibleCorporateEmployeesContributions / us-gaap_TitleOfIndividualAxis = nsp_CorporateEmployeesMember |  |  |  |
Percentage of eligible compensation matched, minimum (in hundredths) | 0.00%nsp_PercentageOfEligibleCompensationMatchedMinimum / us-gaap_TitleOfIndividualAxis = nsp_CorporateEmployeesMember |  |  |  |
Percentage of eligible compensation matched, maximum (in hundredths) | 6.00%nsp_Percentageofeligiblecompensationmatchedmaximum / us-gaap_TitleOfIndividualAxis = nsp_CorporateEmployeesMember |  |  |  |
Corporate Plan and Worksite Employee Plan [Member] |  |  |  |  |
Company-Sponsored 401 (k) Plans [Abstract] |  |  |  |  |
Matching contributions to the Plans | $ 81,500,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_TitleOfIndividualAxis = nsp_CorporatePlanAndWorksiteEmployeePlanMember | $ 74,700,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_TitleOfIndividualAxis = nsp_CorporatePlanAndWorksiteEmployeePlanMember | $ 65,900,000 us-gaap_DefinedContributionPlanCostRecognized / us-gaap_TitleOfIndividualAxis = nsp_CorporatePlanAndWorksiteEmployeePlanMember |  |
Minimum [Member] |  |  |  |  |
Other Intangible Assets [Line Items] |  |  |  |  |
Finite-lived intangible asset, useful life | 3 years |  |  |  |
Maximum [Member] |  |  |  |  |
Other Intangible Assets [Line Items] |  |  |  |  |
Finite-lived intangible asset, useful life | 10 years |  |  |  |
Buildings and Improvements [Member] | Minimum [Member] |  |  |  |  |
Property and Equipment, Estimated Useful Lives [Abstract] |  |  |  |  |
Estimated useful lives of property and equipment | 5 years |  |  |  |
Buildings and Improvements [Member] | Maximum [Member] |  |  |  |  |
Property and Equipment, Estimated Useful Lives [Abstract] |  |  |  |  |
Estimated useful lives of property and equipment | 30 years |  |  |  |
Computer Hardware and Software and Acquired Technologies [Member] | Minimum [Member] |  |  |  |  |
Property and Equipment, Estimated Useful Lives [Abstract] |  |  |  |  |
Estimated useful lives of property and equipment | 1 year |  |  |  |
Computer Hardware and Software and Acquired Technologies [Member] | Maximum [Member] |  |  |  |  |
Property and Equipment, Estimated Useful Lives [Abstract] |  |  |  |  |
Estimated useful lives of property and equipment | 5 years |  |  |  |
Software Development Costs [Member] | Minimum [Member] |  |  |  |  |
Property and Equipment, Estimated Useful Lives [Abstract] |  |  |  |  |
Estimated useful lives of property and equipment | 3 years |  |  |  |
Software Development Costs [Member] | Maximum [Member] |  |  |  |  |
Property and Equipment, Estimated Useful Lives [Abstract] |  |  |  |  |
Estimated useful lives of property and equipment | 5 years |  |  |  |
Furniture and Fixtures [Member] | Minimum [Member] |  |  |  |  |
Property and Equipment, Estimated Useful Lives [Abstract] |  |  |  |  |
Estimated useful lives of property and equipment | 5 years |  |  |  |
Furniture and Fixtures [Member] | Maximum [Member] |  |  |  |  |
Property and Equipment, Estimated Useful Lives [Abstract] |  |  |  |  |
Estimated useful lives of property and equipment | 7 years |  |  |  |
Aircraft [Member] | Minimum [Member] |  |  |  |  |
Property and Equipment, Estimated Useful Lives [Abstract] |  |  |  |  |
Estimated useful lives of property and equipment | 15 years |  |  |  |
Aircraft [Member] | Maximum [Member] |  |  |  |  |
Property and Equipment, Estimated Useful Lives [Abstract] |  |  |  |  |
Estimated useful lives of property and equipment | 20 years |  |  |  |

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- Definition The decrease or increase in workers' compensation claims cost estimates for changes in the actuarial assumptions resulting from changes in actual claims experience and other trends. + References No definition available. + Details
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- Definition Carrying value of amounts transferred to third party workers' compensation insurance providers for security purposes that are expected to be returned or applied towards payment after one year or beyond the operating cycle, if longer. + References No definition available. + Details
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- Definition The number of days in advance of the beginning of a reporting quarter that United Healthcare establishes cash funding rates. + References No definition available. + Details
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- Definition The number of stock-based employee compensation plans the entity has in place at period end. + References No definition available. + Details
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- Definition Number of years in transition from a professional employer organization to a comprehensive business performance solutions provider. + References No definition available. + Details
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- Definition The liability as of the balance sheet date representing required funding mandated by statute or regulatory authority that is related directly or indirectly to underwriting activities, including pools for self-insurance and excluding premium taxes and income taxes. The funds derived from the assessments generally enable a government guarantor to pay insurance claims in the event that the underwriting insurer is insolvent or otherwise unable to meet its payment obligations. + References No definition available. + Details
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- Definition U.S. Treasury rates that correspond with the weighted average estimated claim payout period utilized to discount workers' compensation cost estimates. + References No definition available. + Details
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- Definition The vesting period for restricted stock awards granted to directors upon their initial appointment to the Board. + References No definition available. + Details
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- Definition The change in worker's compensation reserve liability during the period resulting from the discount applied to reduce the reserve to present value. + References No definition available. + Details
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- Definition Carrying value, net of administrative fees, as of the balance sheet date of obligations and payables pertaining to claims incurred of a workers compensation nature. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer). + References No definition available. + Details
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- Definition The maximum amount the Company will bear of the economic burden per policy year for those claims exceeding the first layer maximum per claim occurrence. + References No definition available. + Details
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- Definition The Company's maximum economic burden for the first layer of claims per occurrence. + References No definition available. + Details
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