Segment Information - Certain Operating Data for Segments (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Net sales	$ 4,447.3	$ 4,620.0	$ 8,871.0	$ 8,947.4
Gain on sale of certain closed facilities	5.0		5.5	50.5
Multiemployer pension withdrawal expense	(0.9)		(0.9)
Land and Development impairments		(13.0)		(13.0)
Restructuring and other costs	(16.4)	(34.8)	(46.5)	(89.2)
Interest expense, net	(97.3)	(111.8)	(190.8)	(206.2)
(Loss) gain on extinguishment of debt	(0.5)	0.4	(0.5)	(1.5)
Other expense, net	(0.9)	(3.3)	(4.6)	(6.0)
Income before income taxes	206.7	209.1	392.7	411.6
Depreciation, depletion and amortization	374.5	383.7	755.7	742.8
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	4,498.8	4,659.8	8,964.1	9,026.3
Segment income	335.3	396.0	666.3	720.4
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Net sales	(51.5)	(39.8)	(93.1)	(78.9)
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Non-allocated expenses	(17.6)	(24.4)	(35.8)	(43.4)
Interest expense, net	(97.3)	(111.8)	(190.8)	(206.2)
(Loss) gain on extinguishment of debt	(0.5)	0.4	(0.5)	(1.5)
Other expense, net	(0.9)	(3.3)	(4.6)	(6.0)
Corrugated Packaging [Member]
Segment Reporting Information [Line Items]
Net sales	2,882.5	2,990.7	5,792.0	5,724.5
Depreciation, depletion and amortization	239.6	242.7	483.9	461.2
Corrugated Packaging [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	2,882.5	2,990.7	5,792.0	5,724.5
Segment income	244.5	310.3	527.9	557.1
Corrugated Packaging [Member] \| Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Net sales	(20.7)	(19.4)	(39.0)	(37.4)
Corrugated Packaging [Member] \| Unaffiliated Customers [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	2,861.8	2,971.3	5,753.0	5,687.1
Consumer Packaging [Member]
Segment Reporting Information [Line Items]
Net sales	1,616.3	1,668.3	3,153.2	3,287.1
Depreciation, depletion and amortization	133.2	139.2	268.5	277.8
Consumer Packaging [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	1,616.3	1,668.3	3,153.2	3,287.1
Segment income	90.8	85.2	137.0	162.1
Consumer Packaging [Member] \| Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Net sales	(30.8)	(20.4)	(54.1)	(41.5)
Consumer Packaging [Member] \| Unaffiliated Customers [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	1,585.5	1,647.9	3,099.1	3,245.6
Land and Development [Member]
Segment Reporting Information [Line Items]
Net sales		0.8	18.9	14.7
Land and Development [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales		0.8	18.9	14.7
Segment income		0.5	1.4	1.2
Land and Development [Member] \| Unaffiliated Customers [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales		0.8	18.9	14.7
Corporate Segment [Member]
Segment Reporting Information [Line Items]
Depreciation, depletion and amortization	$ 1.7	$ 1.8	$ 3.3	$ 3.8

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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