Segment Information - Changes in Carrying Amount of Goodwill (Details) $ in Millions	6 Months Ended
	Mar. 31, 2022 USD ($)
Goodwill [Line Items]
Balance at beginning of period	$ 5,959.2
Goodwill acquired	3.2
Translation adjustments	6.9
Balance at end of period	5,969.3
Legacy Reportable Corrugated Packaging Segment [Member]
Goodwill [Line Items]
Balance at beginning of period	3,663.3
Segment recasting	(3,663.3)	[1]
Legacy Reportable Consumer Packaging Segment [Member]
Goodwill [Line Items]
Balance at beginning of period	2,295.9
Segment recasting	(2,295.9)	[1]
New Reportable Corrugated Packaging Segment [Member]
Goodwill [Line Items]
Segment recasting	2,834.8	[1]
Goodwill acquired	3.2
Translation adjustments	0.1
Balance at end of period	2,838.1
New Reportable Consumer Packaging Segment [Member]
Goodwill [Line Items]
Segment recasting	1,603.3	[1]
Translation adjustments	5.0
Balance at end of period	1,608.3
New Reportable Paper Segment [Member]
Goodwill [Line Items]
Segment recasting	1,382.0	[1]
Translation adjustments	1.6
Balance at end of period	1,383.6
New Reportable Distribution Segment [Member]
Goodwill [Line Items]
Segment recasting	139.1	[1]
Translation adjustments	0.2
Balance at end of period	$ 139.3

[1] Represents reallocation of goodwill as a result of changes in segments on October 1, 2021.