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CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) (Parenthetical) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
Gross billings	$ 4,898	$ 4,314	$ 14,656	$ 13,040
Worksite employee payroll costs	$ 4,102	$ 3,611	$ 12,182	$ 10,828

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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.

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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.

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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