Restructuring and Other Costs - Schedule of Restructuring Charges Related to Active Restructuring Initiatives (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Restructuring Cost And Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	$ 62.7	$ 15.4
Restructuring and Related Cost, Cost Incurred to Date	1,303.3
Restructuring and Related Cost, Expected Cost	1,442.4
PP&E and Related Costs [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	(3.1)	(1.6)
Restructuring and Related Cost, Cost Incurred to Date	971.9
Restructuring and Related Cost, Expected Cost	971.9
Severance and Other Employee Costs [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	18.2	10.1
Restructuring and Related Cost, Cost Incurred to Date	115.6
Restructuring and Related Cost, Expected Cost	118.2
Other Restructuring Costs [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	47.6	6.9
Restructuring and Related Cost, Cost Incurred to Date	215.8
Restructuring and Related Cost, Expected Cost	352.3
Corrugated Packaging [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	8.0	0.9
Restructuring and Related Cost, Cost Incurred to Date	46.8
Restructuring and Related Cost, Expected Cost	69.0
Corrugated Packaging [Member] | PP&E and Related Costs [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	0.3	(0.9)
Restructuring and Related Cost, Cost Incurred to Date	12.7
Restructuring and Related Cost, Expected Cost	12.7
Corrugated Packaging [Member] | Severance and Other Employee Costs [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	4.5	1.7
Restructuring and Related Cost, Cost Incurred to Date	23.2
Restructuring and Related Cost, Expected Cost	23.3
Corrugated Packaging [Member] | Other Restructuring Costs [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	3.2	0.1
Restructuring and Related Cost, Cost Incurred to Date	10.9
Restructuring and Related Cost, Expected Cost	33.0
Consumer Packaging [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	20.9	6.1
Restructuring and Related Cost, Cost Incurred to Date	59.6
Restructuring and Related Cost, Expected Cost	69.9
Consumer Packaging [Member] | PP&E and Related Costs [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	0.7
Restructuring and Related Cost, Cost Incurred to Date	5.0
Restructuring and Related Cost, Expected Cost	5.0
Consumer Packaging [Member] | Severance and Other Employee Costs [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	18.6	6.4
Restructuring and Related Cost, Cost Incurred to Date	46.3
Restructuring and Related Cost, Expected Cost	47.8
Consumer Packaging [Member] | Other Restructuring Costs [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	1.6	(0.3)
Restructuring and Related Cost, Cost Incurred to Date	8.3
Restructuring and Related Cost, Expected Cost	17.1
Global Paper [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	34.8	4.7
Restructuring and Related Cost, Cost Incurred to Date	1,157.3
Restructuring and Related Cost, Expected Cost	1,257.5
Global Paper [Member] | PP&E and Related Costs [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	(4.1)	(1.1)
Restructuring and Related Cost, Cost Incurred to Date	951.6
Restructuring and Related Cost, Expected Cost	951.6
Global Paper [Member] | Severance and Other Employee Costs [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	(5.1)	(0.2)
Restructuring and Related Cost, Cost Incurred to Date	36.9
Restructuring and Related Cost, Expected Cost	37.9
Global Paper [Member] | Other Restructuring Costs [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	44.0	6.0
Restructuring and Related Cost, Cost Incurred to Date	168.8
Restructuring and Related Cost, Expected Cost	268.0
Distribution [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	(2.9)
Restructuring and Related Cost, Cost Incurred to Date	9.9
Restructuring and Related Cost, Expected Cost	12.2
Distribution [Member] | Severance and Other Employee Costs [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	(0.1)
Restructuring and Related Cost, Cost Incurred to Date	1.7
Restructuring and Related Cost, Expected Cost	1.7
Distribution [Member] | Other Restructuring Costs [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	(2.8)
Restructuring and Related Cost, Cost Incurred to Date	8.2
Restructuring and Related Cost, Expected Cost	10.5
Corporate, Non-Segment [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	1.9	3.7
Restructuring and Related Cost, Cost Incurred to Date	29.7
Restructuring and Related Cost, Expected Cost	33.8
Corporate, Non-Segment [Member] | PP&E and Related Costs [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost		0.4
Restructuring and Related Cost, Cost Incurred to Date	2.6
Restructuring and Related Cost, Expected Cost	2.6
Corporate, Non-Segment [Member] | Severance and Other Employee Costs [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	0.3	2.2
Restructuring and Related Cost, Cost Incurred to Date	7.5
Restructuring and Related Cost, Expected Cost	7.5
Corporate, Non-Segment [Member] | Other Restructuring Costs [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost	1.6	$ 1.1
Restructuring and Related Cost, Cost Incurred to Date	19.6
Restructuring and Related Cost, Expected Cost	$ 23.7