REVENUE FROM CONTRACTS WITH CUSTOMERS (Details) $ in Thousands	6 Months Ended
Mar. 27, 2026 USD ($) performance_obligation
Disaggregation of Revenue [Line Items]
Number of performance obligations \| performance_obligation	2
Reduction of revenue for economic value of tax credits to be transferred	$ 30,391
Benefit to cost of sales for the economic value of tax credits to be transferred	33,993
Liability for credits to be transferred	$ 18,167
Minimum
Disaggregation of Revenue [Line Items]
Period of payment received from related performance obligation satisfied	30 days
Maximum
Disaggregation of Revenue [Line Items]
Period of payment received from related performance obligation satisfied	60 days
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-03-28
Disaggregation of Revenue [Line Items]
Revenue, remaining performance obligation, expected timing of satisfaction, period	1 year

Increase (Decrease) In Cost Of Sales, Economic Value Of Tax Credits To Be Transferred

No definition available.

Name:	atkr_IncreaseDecreaseInCostOfSalesEconomicValueOfTaxCreditsToBeTransferred
Namespace Prefix:	atkr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Increase (Decrease) Of Revenue, Economic Value Of Tax Credits To Be Transferred

+ References

No definition available.

+ Details

Name:	atkr_IncreaseDecreaseOfRevenueEconomicValueOfTaxCreditsToBeTransferred
Namespace Prefix:	atkr_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Liability For Credits To Be Transferred

+ References

No definition available.

+ Details

Name:	atkr_LiabilityForCreditsToBeTransferred
Namespace Prefix:	atkr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Revenue From Contract With Customer, Payment Term

+ References

No definition available.

+ Details

Name:	atkr_RevenueFromContractWithCustomerPaymentTerm
Namespace Prefix:	atkr_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Revenue, Number Of Performance Obligations

+ References

No definition available.

+ Details

Name:	atkr_RevenueNumberOfPerformanceObligations
Namespace Prefix:	atkr_
Data Type:	xbrli:integerItemType
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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-7

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-5

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 91
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479777/606-10-55-91

Reference 4: http://www.xbrl.org/2003/role/exampleRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 91
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479777/606-10-55-91

Reference 5: http://www.xbrl.org/2003/role/exampleRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 91
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479777/606-10-55-91

Reference 6: http://www.xbrl.org/2003/role/exampleRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 91
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479777/606-10-55-91

Reference 7: http://www.xbrl.org/2003/role/exampleRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 91
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479777/606-10-55-91

Reference 8: http://www.xbrl.org/2003/role/exampleRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 91
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479777/606-10-55-91

Reference 9: http://www.xbrl.org/2003/role/exampleRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 91
-Subparagraph (g)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479777/606-10-55-91

+ Details

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

- Definition

Period in which remaining performance obligation is expected to be recognized as revenue, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)(1)
-SubTopic 10
-Topic 606
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-13

+ Details

Name:	us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis=2026-03-28
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: