Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2020	Oct. 26, 2020
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2020
Document Transition Report	false
Entity File Number	1-13998
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	76-0479645
Entity Address, Address Line One	19001 Crescent Springs Drive
Entity Address, City or Town	Kingwood,
Entity Address, State or Province	TX
Entity Address, Postal Zip Code	77339
City Area Code	281
Local Phone Number	358-8986
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		38,354,147
Entity Registrant Name	INSPERITY, INC.
Entity Central Index Key	0001000753
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3
Amendment Flag	false
Common Stock [Member]
Entity Information [Line Items]
Title of 12(b) Security	Common Stock, $.01 par value per share
Trading Symbol	NSP
Security Exchange Name	NYSE
Rights [Member]
Entity Information [Line Items]
Title of 12(b) Security	Rights to Purchase Series A Junior Participating Preferred Stock
Trading Symbol	NSP
Security Exchange Name	NYSE