Revenues - Schedule of Changes in Estimated Contract Earnings (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
Jan. 02, 2026	Dec. 27, 2024
Revenue from Contract with Customer [Abstract]
Favorable earnings at completion adjustments	$ 47	$ 32
Unfavorable earnings at completion adjustments	(24)	(24)
Net favorable adjustments	$ 23	$ 8
Impact on diluted earnings per share attributable to common shareholders (in dollars per share)	$ 0.07	$ 0.02

Contract With Customer Asset, Cumulative Catch Up Adjustment To Revenue, Favorable Adjustment

No definition available.

Name:	amtm_ContractWithCustomerAssetCumulativeCatchUpAdjustmentToRevenueFavorableAdjustment
Namespace Prefix:	amtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Contract With Customer Asset, Cumulative Catch Up Adjustment To Revenue, Unfavorable Adjustment

+ References

No definition available.

+ Details

Name:	amtm_ContractWithCustomerAssetCumulativeCatchUpAdjustmentToRevenueUnfavorableAdjustment
Namespace Prefix:	amtm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Contract With Customer, Asset, Effect On Diluted Earnings (Loss) Per Share

+ References

No definition available.

+ Details

Name:	amtm_ContractWithCustomerAssetEffectOnDilutedEarningsLossPerShare
Namespace Prefix:	amtm_
Data Type:	dtr-types:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of increase (decrease) in revenue recognized for cumulative catch-up adjustment from change in estimate of transaction price which increases (decreases) right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time. Includes, but is not limited to, change in assessment of whether estimate of variable consideration is constrained.

+ References

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

+ Details

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

- References

No definition available.

+ Details

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