CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) (Parenthetical) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Income Statement [Abstract]
Gross billings	$ 3,703	$ 3,281	$ 7,643	$ 6,869
Worksite employee payroll costs	$ 3,075	$ 2,716	$ 6,316	$ 5,667

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