Segment Information - Changes in Carrying Amount of Goodwill (Details) $ in Millions	3 Months Ended	6 Months Ended
	Mar. 31, 2023 USD ($)	Mar. 31, 2023 USD ($)
Goodwill [Line Items]
Balance at beginning of period		$ 5,895.2
Goodwill impairment	$ (1,893.0)	(1,893.0)
Acquisitions		233.3
Divestitures		(11.5)
Transferred to assets held for sale		(24.9)
Translation and other adjustments		53.9
Goodwill	6,146.0	6,146.0
Accumulated impairment losses	(1,893.0)	(1,893.0)
Balance at end of period	4,253.0	4,253.0
Corrugated Packaging
Goodwill [Line Items]
Balance at beginning of period		2,802.8
Goodwill impairment		(514.3)
Acquisitions		233.3
Translation and other adjustments		71.9
Goodwill	3,108.0	3,108.0
Accumulated impairment losses	(514.3)	(514.3)
Balance at end of period	2,593.7	2,593.7
Consumer Packaging
Goodwill [Line Items]
Balance at beginning of period		1,588.4
Divestitures		(7.4)
Transferred to assets held for sale		(24.9)
Translation and other adjustments		(35.4)
Goodwill	1,520.7	1,520.7
Balance at end of period	1,520.7	1,520.7
Global Paper
Goodwill [Line Items]
Balance at beginning of period		1,366.5
Goodwill impairment		(1,378.7)
Divestitures		(4.1)
Translation and other adjustments		16.3
Goodwill	1,378.7	1,378.7
Accumulated impairment losses	(1,378.7)	(1,378.7)
Distribution
Goodwill [Line Items]
Balance at beginning of period		137.5
Translation and other adjustments		1.1
Goodwill	138.6	138.6
Balance at end of period	$ 138.6	$ 138.6