Segment Information - Certain Financial Data for Segments (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Sep. 30, 2023
Segment Reporting Information [Line Items]
Net sales	$ 4,620.0	$ 4,923.1
Depreciation, depletion and amortization	(381.8)	(373.2)
Restructuring and other costs, net	(65.5)	(32.1)
Non-allocated expenses	(50.7)	(28.7)
Interest expense, net	(101.4)	(97.3)
Other (expense) income, net	(4.7)	25.2
Gain on sale of RTS and Chattanooga	0.5
Other adjustments	(34.8)	(119.6)
(Loss) income before income taxes	(17.0)	55.1
Depreciation, depletion and amortization	381.8	373.2
Other adjustments	34.8	119.6
Equity in income (loss) of unconsolidated entities	4.2	(36.0)
Assets	26,746.2		$ 27,443.7
Intangibles, net	2,507.3		$ 2,576.2
Capital expenditures	247.3	282.2
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	4,687.2	4,997.5
Adjusted EBITDA	621.4	680.8
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Net sales	(67.2)	(74.4)
Corrugated Packaging [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	2,419.9	2,337.4
Adjusted EBITDA	327.8	329.4
Depreciation, depletion and amortization	205.3	192.2
Other adjustments	5.1	49.8
Equity in income (loss) of unconsolidated entities	3.2	(35.8)
Corrugated Packaging [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Net sales	(59.2)	(68.4)
Corrugated Packaging [Member] | Unaffiliated Customers [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	2,360.7	2,269.0
Consumer Packaging [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	1,059.3	1,215.0
Adjusted EBITDA	166.2	183.3
Depreciation, depletion and amortization	86.5	84.1
Other adjustments	3.6	31.6
Consumer Packaging [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Net sales	(5.3)	(4.7)
Consumer Packaging [Member] | Unaffiliated Customers [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	1,054.0	1,210.3
Global Paper [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	918.3	1,123.6
Adjusted EBITDA	118.4	157.3
Depreciation, depletion and amortization	81.1	89.1
Other adjustments	1.5	17.5
Equity in income (loss) of unconsolidated entities	1.0	(0.2)
Global Paper [Member] | Unaffiliated Customers [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	918.3	1,123.6
Distribution [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	289.7	321.5
Adjusted EBITDA	9.0	10.8
Depreciation, depletion and amortization	7.3	6.9
Other adjustments	(0.3)
Distribution [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Net sales	(2.7)	(1.3)
Distribution [Member] | Unaffiliated Customers [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	287.0	320.2
Corporate Segment [Member]
Segment Reporting Information [Line Items]
Depreciation, depletion and amortization	1.6	0.9
Other adjustments	$ 24.9	$ 20.7