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CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) (Parenthetical) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2014 | Jun. 30, 2013 | Jun. 30, 2014 | Jun. 30, 2013
Income Statement [Abstract] |  |  |  |
Gross billings | $ 3,281 | $ 3,167 | $ 6,869 | $ 6,499
Worksite employee payroll costs | $ 2,716 | $ 2,620 | $ 5,667 | $ 5,340

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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
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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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- Details
Name: | us-gaap_IncomeStatementAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration