CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) (Parenthetical) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Income Statement [Abstract]
Gross billings	$ 3,940nsp_GrossBillings	$ 3,588nsp_GrossBillings
Worksite employee payroll costs	$ 3,241nsp_WorksiteEmployeePayrollCost	$ 2,951nsp_WorksiteEmployeePayrollCost

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.

No definition available.

Name:	nsp_WorksiteEmployeePayrollCost
Namespace Prefix:	nsp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration