Cover Page - shares	6 Months Ended
	Jun. 27, 2021	Jul. 27, 2021
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Jun. 27, 2021
Document Transition Report	false
Entity File Number	001-38580
Entity Registrant Name	IAA, Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	83-1030538
Entity Address, Address Line One	Two Westbrook Corporate Center
Entity Address, Address Line Two	Suite 500
Entity Address, City or Town	Westchester
Entity Address, State or Province	IL
Entity Address, Postal Zip Code	60154
City Area Code	708
Local Phone Number	492-7000
Title of 12(b) Security	Common Stock, par value $0.01 per share
Trading Symbol	IAA
Security Exchange Name	NYSE
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		134,796,790
Entity Central Index Key	0001745041
Current Fiscal Year End Date	--01-02
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q2
Amendment Flag	false