CONSOLIDATED STATEMENTS OF OPERATIONS (Parenthetical) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
CONSOLIDATED STATEMENTS OF OPERATIONS [Abstract]
Gross billings	$ 12,992	$ 11,700	$ 10,169
Worksite employee payroll cost	$ 10,833	$ 9,724	$ 8,449

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