CONSOLIDATED STATEMENTS OF OPERATIONS (Parenthetical) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Gross billings	$ 14,187nsp_GrossBillings	$ 13,462nsp_GrossBillings	$ 12,992nsp_GrossBillings
Worksite employee payroll costs	$ 11,829nsp_WorksiteEmployeePayrollCost	$ 11,206nsp_WorksiteEmployeePayrollCost	$ 10,833nsp_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