Debt Instrument, Covenant, Net Leverage Ratio, Maximum

No definition available.

Name:	iaa_DebtInstrumentCovenantNetLeverageRatioMaximum
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- Definition

Letters of Credit Outstanding, Amount Reduced Under Credit Facility

+ References

No definition available.

+ Details

Name:	iaa_LettersofCreditOutstandingAmountReducedUnderCreditFacility
Namespace Prefix:	iaa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line Of Credit Facility, Sub-Limit For Issuance Of Letters Of Credit

+ References

No definition available.

+ Details

Name:	iaa_LineOfCreditFacilitySubLimitForIssuanceOfLettersOfCredit
Namespace Prefix:	iaa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line Of Credit Facility, Sub-Limit For Swing Line Loans

+ References

No definition available.

+ Details

Name:	iaa_LineOfCreditFacilitySubLimitForSwingLineLoans
Namespace Prefix:	iaa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line of Credit Facility, Maximum Borrowing Capacity, Increase Amount

+ References

No definition available.

+ Details

Name:	iaa_LineofCreditFacilityMaximumBorrowingCapacityIncreaseAmount
Namespace Prefix:	iaa_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Number of Promissory Notes

+ References

No definition available.

+ Details

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Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
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+ Details

Name:	us-gaap_DebtInstrumentCarryingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
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+ Details

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (a)
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+ Details

Name:	us-gaap_DebtInstrumentInterestRateEffectivePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Weighted average interest rate of debt outstanding.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtWeightedAverageInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

Name:	us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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+ Details

Name:	us-gaap_LineOfCredit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The fee, expressed as a percentage of the line of credit facility, for the line of credit facility regardless of whether the facility has been used.

+ References

No definition available.

+ Details

Name:	us-gaap_LineOfCreditFacilityCommitmentFeePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

No definition available.

+ Details

Name:	us-gaap_LongTermDebtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Including the current and noncurrent portions, aggregate carrying amount of all types of notes payable, as of the balance sheet date, with initial maturities beyond one year or beyond the normal operating cycle, if longer.

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+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The cash outflow for debt initially having maturity due after one year or beyond the normal operating cycle, if longer.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3291-108585

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SeniorNotesMember
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Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_NotesPayableOtherPayablesMember
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- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=srt_AffiliatedEntityMember
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Period Type:

- Details

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Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=iaa_SeniorNotesdue20175.500Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_SecuredDebtMember
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Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=iaa_SeniorSecuredTermLoanFacilitySevenYearMember
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Data Type:	na
Balance Type:
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- Details

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Long-Term Debt - Narrative (Details)		6 Months Ended
Long-Term Debt - Narrative (Details)	Jun. 28, 2019 USD ($)	Jun. 28, 2020 USD ($)	Jun. 30, 2019 USD ($)	May 01, 2020 USD ($)	Dec. 29, 2019 USD ($)	Jun. 06, 2019 USD ($)	Dec. 30, 2018 USD ($) note
Long-Term Debt
Optional principal pre-payment		$ 4,000,000.0	$ 0
Total debt		1,274,000,000.0			$ 1,278,000,000.0
Outstanding letters of credit		6,500,000			7,000,000.0
Amount reduced under Revolving Credit Facility		6,100,000
Estimated fair value of long-term debt		1,264,400,000
Term Loan Facility
Long-Term Debt
Optional principal pre-payment		4,000,000.0
Total debt		774,000,000.0			778,000,000.0
Senior Notes
Long-Term Debt
Total debt		$ 500,000,000.0			$ 500,000,000.0
Promissory Notes \| Affiliated Entity
Long-Term Debt
Intercompany debt							$ 456,600,000
Number of promissory notes \| note							3
Weighted average interest rate							8.27%
Credit Agreement
Long-Term Debt
Net leverage ratio	3.50
Notes \| Senior Notes
Long-Term Debt
Aggregate principal amount, senior notes						$ 500,000,000.0
Senior notes stated interest rate, percentage						5.50%
Secured Debt \| Term Loan Facility
Long-Term Debt
Credit facility, term	7 years
Aggregate principal amount, line of credit	$ 800,000,000
Secured Debt \| Term Loan Facility \| Base Rate
Long-Term Debt
Interest rate, per annum		2.40%			4.00%
Revolving Credit Facility \| Revolving Credit Facility
Long-Term Debt
Credit facility, term	5 years
Aggregate principal amount, line of credit	$ 225,000,000
Sub-limit for issuance of letters of credit				$ 50,000,000
Sub-limit for swing line loans				50,000,000
Amount outstanding		$ 0
Revolving Credit Facility \| Revolving Credit Facility \| Minimum
Long-Term Debt
Commitment fee	0.25%
Revolving Credit Facility \| Revolving Credit Facility \| Maximum
Long-Term Debt
Commitment fee	0.35%
Revolving Credit Facility \| Credit Agreement Amendment
Long-Term Debt
Aggregate principal amount, line of credit				361,000,000.0
Increase in aggregate principal amount				$ 136,000,000.0