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Retirement Plans (Details) $ in Millions	12 Months Ended
	Sep. 27, 2024 USD ($) plan	Sep. 29, 2023 USD ($)	Sep. 30, 2022 USD ($)
Retirement Benefits [Abstract]
Number of participant-directed | plan	1
Total retirement plan expense | $	$ 55	$ 54	$ 53

X

- Definition

Defined Contribution Plan, Number Of Plans

+ References

No definition available.

+ Details

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Namespace Prefix:	amtm_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

X

- References

No definition available.

+ Details

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

X

- Definition

Amount of cost for defined contribution plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 715
-SubTopic 70
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480794/715-70-50-1

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	debit
Period Type:	duration