CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) (Parenthetical) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012
CONSOLIDATED STATEMENTS OF OPERATIONS [Abstract]
Gross billings	$ 3,332	$ 3,231
Worksite employee payroll cost	$ 2,720	$ 2,636

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