CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) (Parenthetical) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
CONSOLIDATED STATEMENTS OF OPERATIONS [Abstract]
Gross billings	$ 3,068	$ 2,835	$ 9,339	$ 8,454
Worksite employee payroll cost	$ 2,556	$ 2,363	$ 7,712	$ 6,973

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