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Segment Information - Certain Operating Data for Segments (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2019	Jun. 30, 2018	Sep. 30, 2018
Segment Reporting Information [Line Items]
Net sales	$ 4,690.0		$ 4,137.5		$ 13,637.4	$ 12,048.5
Multiemployer pension withdrawals	1.7		(4.2)		1.7	(184.2)
Land and Development impairments		$ (13.0)	(1.7)	$ (27.6)	(13.0)	(29.3)
Restructuring and other costs	(17.9)		(17.1)		(107.1)	(65.1)
Interest expense, net	(111.1)		(76.7)		(317.3)	(219.8)
(Loss) gain on extinguishment of debt	(3.2)		0.9		(4.7)
Other income (expense), net	3.7		9.7		(2.3)	13.3
Income before income taxes	331.4		355.8		743.0	659.4
Intangibles, net	4,174.8				4,174.8		$ 3,122.0
Identifiable assets	30,812.1				30,812.1		25,360.5
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	4,731.5		4,179.0		13,757.8	12,166.7
Segment income	485.3		457.9		1,205.7	1,194.8
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Net sales	(41.5)		(41.5)		(120.4)	(118.2)
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
(Loss) gain on sale of certain closed facilities	(2.7)				47.8
Multiemployer pension withdrawals	1.7		(4.2)		1.7	(184.2)
Land and Development impairments			(1.7)		(13.0)	(29.3)
Non-allocated expenses	(24.4)		(13.0)		(67.8)	(50.3)
Interest expense, net	(111.1)		(76.7)		(317.3)	(219.8)
(Loss) gain on extinguishment of debt	(3.2)		0.9		(4.7)
Other income (expense), net	3.7		9.7		(2.3)	13.3
Corrugated Packaging [Member]
Segment Reporting Information [Line Items]
Net sales	3,072.8				8,797.3
Intangibles, net	1,711.5				1,711.5		506.2
Identifiable assets	16,893.5				16,893.5		11,069.6
Corrugated Packaging [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	3,072.8		2,444.6		8,797.3	7,155.6
Segment income	392.7		321.9		949.8	854.6
Corrugated Packaging [Member] \| Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Net sales	(19.7)		(21.6)		(57.1)	(63.0)
Corrugated Packaging [Member] \| Unaffiliated Customers [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	3,053.1		2,423.0		8,740.2	7,092.6
Consumer Packaging [Member]
Segment Reporting Information [Line Items]
Net sales	1,650.1				4,937.2
Intangibles, net	2,463.3				2,463.3		2,615.8
Identifiable assets	11,216.9				11,216.9		11,511.1
Consumer Packaging [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	1,650.1		1,669.6		4,937.2	4,908.2
Segment income	91.0		126.1		253.1	314.9
Consumer Packaging [Member] \| Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Net sales	(21.8)		(19.9)		(63.3)	(55.2)
Consumer Packaging [Member] \| Unaffiliated Customers [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	1,628.3		1,649.7		4,873.9	4,853.0
Land and Development [Member]
Segment Reporting Information [Line Items]
Net sales	8.6				23.3
Identifiable assets	27.4				27.4		49.1
Land and Development [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	8.6		64.8		23.3	102.9
Segment income	1.6		9.9		2.8	25.3
Land and Development [Member] \| Unaffiliated Customers [Member] \| Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	8.6		$ 64.8		23.3	$ 102.9
Disposal Group, Held-for-sale, Not Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Identifiable assets	26.6				26.6		59.5
Corporate Segment [Member]
Segment Reporting Information [Line Items]
Identifiable assets	$ 2,647.7				$ 2,647.7		$ 2,671.2