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CONSOLIDATED STATEMENTS OF OPERATIONS (Parenthetical) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Income Statement [Abstract] |  |  |
Gross billings | $ 23,830,731 | $ 20,173,812 | $ 17,932,857
Worksite employee payroll costs | $ 20,002,182 | $ 16,873,589 | $ 14,991,510

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- Definition The company's revenues are derived from its gross billings, which are based on the payroll cost of its worksite employees and a markup computed as a percentage of the payroll costs. + References No definition available. + Details
Name: | nsp_GrossBillings
Namespace Prefix: | nsp_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Payroll costs incurred (including share-based compensation) by the company's worksite employees that are directly related to services rendered by the worksite employees during the reporting period. + References No definition available. + Details
Name: | nsp_WorksiteEmployeePayrollCost
Namespace Prefix: | nsp_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_IncomeStatementAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration