Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Oct. 25, 2018
Document and Entity Information [Abstract]
Entity Registrant Name	INSPERITY, INC.
Entity Central Index Key	0001000753
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Common Stock, Shares Outstanding		41,810,065
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q3
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2018