CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) (Parenthetical) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Income Statement [Abstract]
Gross billings	$ 5,923	$ 5,016
Worksite employee payroll costs	$ 4,909	$ 4,133

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