Segment Information - Certain Operating Data for Segments (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Net sales	$ 4,236.3	$ 4,690.0	$ 13,107.3	$ 13,637.4
(Loss) gain on sale of certain closed facilities		(2.7)	5.5	47.8
Multiemployer pension withdrawal income	2.0	1.7	1.1	1.7
Land and Development impairments				(13.0)
Restructuring and other costs	(9.7)	(17.9)	(56.2)	(107.1)
Interest expense, net	(92.4)	(111.1)	(283.2)	(317.3)
Loss on extinguishment of debt	(0.6)	(3.2)	(1.1)	(4.7)
Other (expense) income, net	(5.0)	3.7	(9.6)	(2.3)
Income before income taxes	199.2	331.4	591.9	743.0
Depreciation, depletion and amortization	365.7	385.3	1,121.4	1,128.1
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	4,281.4	4,731.5	13,245.5	13,757.8
Segment income	323.2	485.3	989.5	1,205.7
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Net sales	(45.1)	(41.5)	(138.2)	(120.4)
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Non-allocated expenses	(18.3)	(24.4)	(54.1)	(67.8)
Interest expense, net	(92.4)	(111.1)	(283.2)	(317.3)
Corrugated Packaging [Member]
Segment Reporting Information [Line Items]
Net sales	2,728.8	3,072.8	8,520.8	8,797.3
Depreciation, depletion and amortization	233.1	241.4	717.0	702.6
Corrugated Packaging [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	2,728.8	3,072.8	8,520.8	8,797.3
Segment income	227.9	392.7	755.8	949.8
Corrugated Packaging [Member] \| Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Net sales	(15.3)	(19.7)	(54.3)	(57.1)
Corrugated Packaging [Member] \| Unaffiliated Customers [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	2,713.5	3,053.1	8,466.5	8,740.2
Consumer Packaging [Member]
Segment Reporting Information [Line Items]
Net sales	1,552.6	1,650.1	4,705.8	4,937.2
Depreciation, depletion and amortization	131.2	140.7	399.7	418.5
Consumer Packaging [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	1,552.6	1,650.1	4,705.8	4,937.2
Segment income	95.3	91.0	232.3	253.1
Consumer Packaging [Member] \| Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Net sales	(29.8)	(21.8)	(83.9)	(63.3)
Consumer Packaging [Member] \| Unaffiliated Customers [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	1,522.8	1,628.3	4,621.9	4,873.9
Land and Development [Member]
Segment Reporting Information [Line Items]
Net sales		8.6	18.9	23.3
Land and Development [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales		8.6	18.9	23.3
Segment income		1.6	1.4	2.8
Land and Development [Member] \| Unaffiliated Customers [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales		8.6	18.9	23.3
Corporate Segment [Member]
Segment Reporting Information [Line Items]
Depreciation, depletion and amortization	$ 1.4	$ 3.2	$ 4.7	$ 7.0

The aggregate expense recognized in the current period that allocates the cost of tangible assets, intangible assets, or depleting assets to periods that benefit from use of the assets.

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