Financial instruments and Retirement benefit obligations - Retirement benefit obligations (Details) - USD ($) $ in Millions	6 Months Ended
Jun. 27, 2026	Jun. 28, 2025
Disclosure of detailed information about financial instruments [line items]
Remeasurement loss	$ 1	$ (4)
UK
Disclosure of detailed information about financial instruments [line items]
Actuarial assumption, increase (decrease) in discount rate during the period	0.50%
Discount rate	6.00%
Germany
Disclosure of detailed information about financial instruments [line items]
Actuarial assumption, increase (decrease) in discount rate during the period	0.20%
Discount rate	4.60%
SWITZERLAND
Disclosure of detailed information about financial instruments [line items]
Actuarial assumption, increase (decrease) in discount rate during the period	(0.05%)
Discount rate	1.15%

The amount of other comprehensive income, before tax, related to gains (losses) on remeasurements of defined benefit plans, which comprise actuarial gains and losses; the return on plan assets, excluding amounts included in net interest on the net defined benefit liability (asset); and any change in the effect of the asset ceiling, excluding amounts included in net interest on the net defined benefit liability (asset). [Refer: Other comprehensive income; Defined benefit plans [domain]; Plan assets [member]; Net defined benefit liability (asset)] [Contrast: Decrease (increase) in net defined benefit liability (asset) resulting from gain (loss) on remeasurement in other comprehensive income]

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 19
-IssueDate 2025-01-01
-Paragraph 135
-Subparagraph b
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=19&code=ifrs-tx-2025-en-r&anchor=para_135_b&doctype=Standard
-URIDate 2025-03-27

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Note Expiry date 2027-01-01
-Name IAS
-Number 1
-IssueDate 2025-01-01
-Paragraph 91
-Subparagraph b
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=1&code=ifrs-tx-2025-en-b&anchor=para_91_b&doctype=Standard
-URIDate 2025-03-27

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Note Expiry date 2027-01-01
-Name IAS
-Number 1
-IssueDate 2025-01-01
-Paragraph 7
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=1&code=ifrs-tx-2025-en-b&anchor=para_7&doctype=Standard
-URIDate 2025-03-27

Name:	ifrs-full_OtherComprehensiveIncomeBeforeTaxGainsLossesOnRemeasurementsOfDefinedBenefitPlans
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration