CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) (Parenthetical) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Income Statement [Abstract]
Gross billings	$ 3,362	$ 3,236	$ 10,231	$ 9,736
Worksite employee payroll costs	$ 2,802	$ 2,696	$ 8,469	$ 8,037

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