Goodwill (Details) - USD ($) $ in Millions	9 Months Ended
Jun. 30, 2026	Sep. 30, 2025
Goodwill
Goodwill (gross)	$ 5,440.8	$ 5,574.5
Accumulated impairment losses	(609.1)	(729.8)
Goodwill	4,831.7	4,844.7
Goodwill from acquisition	0.9
Currency translation adjustment	(13.9)
Goodwill (gross) reclassified to held for sale	(120.7)
Accumulated impairment loss reclassified to held for sale [Line Items]	120.7
Post Consumer Brands
Goodwill
Goodwill (gross)	2,409.5	2,408.7
Accumulated impairment losses	(609.1)	(609.1)
Goodwill	1,800.4	1,799.6
Goodwill from acquisition	0.9
Currency translation adjustment	(0.1)
Goodwill (gross) reclassified to held for sale	0.0
Accumulated impairment loss reclassified to held for sale [Line Items]	0.0
Foodservice
Goodwill
Goodwill (gross)	1,389.2	1,389.2
Accumulated impairment losses	0.0	0.0
Goodwill	1,389.2	1,389.2
Goodwill from acquisition	0.0
Currency translation adjustment	0.0
Goodwill (gross) reclassified to held for sale	0.0
Accumulated impairment loss reclassified to held for sale [Line Items]	0.0
Refrigerated Retail
Goodwill
Goodwill (gross)	714.6	835.3
Accumulated impairment losses	0.0	(120.7)
Goodwill	714.6	714.6
Goodwill from acquisition	0.0
Currency translation adjustment	0.0
Goodwill (gross) reclassified to held for sale	(120.7)
Accumulated impairment loss reclassified to held for sale [Line Items]	120.7
Weetabix
Goodwill
Goodwill (gross)	927.5	941.3
Accumulated impairment losses	0.0	0.0
Goodwill	927.5	$ 941.3
Goodwill from acquisition	0.0
Currency translation adjustment	(13.8)
Goodwill (gross) reclassified to held for sale	0.0
Accumulated impairment loss reclassified to held for sale [Line Items]	$ 0.0

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Amount, after accumulated impairment loss, of asset representing future economic benefit arising from other asset acquired in business combination or from joint venture formation or both, that is not individually identified and separately recognized.

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