Segment Information - Certain Operating Data for Segments (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Net sales	$ 4,471.5	$ 4,236.3	$ 4,447.3	$ 4,423.7	$ 4,651.6	$ 4,690.0	$ 4,620.0	$ 4,327.4	$ 17,578.8	$ 18,289.0	$ 16,285.1
Gain on sale of certain closed facilities									15.6	52.6
Multiemployer pension withdrawal income (expense)		2.0	(0.9)		4.6	1.7			1.1	6.3	(184.2)
Land and Development impairments							(13.0)			(13.0)	(31.9)
Restructuring and other costs	(56.5)	(9.7)	(16.4)	$ (30.1)	(66.6)	(17.9)	(34.8)	(54.4)	(112.7)	(173.7)	(105.4)
Goodwill impairment	(1,333.2)								(1,333.2)
Interest expense, net									(393.5)	(431.3)	(293.8)
Loss on extinguishment of debt	(0.4)	$ (0.6)	$ (0.5)		(0.4)	$ (3.2)	$ 0.4	$ (1.9)	(1.5)	(5.1)	(0.1)
Other income, net									9.5	2.4	12.7
(Loss) income before income taxes									(522.6)	1,144.7	1,034.8
Depreciation, depletion and amortization									1,487.0	1,511.2	1,252.2
Identifiable assets	28,779.7				30,156.7				28,779.7	30,156.7	25,360.5
Goodwill	5,962.2				7,285.6				5,962.2	7,285.6	5,577.6	$ 5,528.3
Intangibles, net	3,667.2				4,059.5				3,667.2	4,059.5	3,122.0
Capital expenditures									978.1	1,369.1	999.9
Equity method investments	429.6				469.1				429.6	469.1	457.8
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales									17,771.1	18,446.1	16,452.9
Segment income									1,362.8	1,790.2	1,707.6
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Net sales									(192.3)	(157.1)	(167.8)
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Non-allocated expenses									(70.7)	(83.7)	(70.1)
Interest expense, net									(393.5)	(431.3)	(293.8)
Corrugated Packaging [Member]
Segment Reporting Information [Line Items]
Net sales									11,419.2	11,816.7
Depreciation, depletion and amortization									951.4	950.6	700.5
Identifiable assets	16,507.0				16,681.1				16,507.0	16,681.1	11,069.6
Goodwill	3,673.5				3,695.0				3,673.5	3,695.0	1,966.7	1,941.5
Intangibles, net	1,423.0				1,655.1				1,423.0	1,655.1	506.2
Capital expenditures									731.1	961.4	657.3
Equity method investments	414.3				457.1				414.3	457.1	455.6
Corrugated Packaging [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales									11,419.2	11,816.7	9,693.0
Segment income									1,037.7	1,399.6	1,240.0
Corrugated Packaging [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Net sales									(71.0)	(75.3)	(87.3)
Corrugated Packaging [Member] | Unaffiliated Customers [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales									11,348.2	11,741.4	9,605.7
Consumer Packaging [Member]
Segment Reporting Information [Line Items]
Net sales									6,333.0	6,606.0
Goodwill impairment									(1,333.2)
Depreciation, depletion and amortization									529.5	552.1	546.5
Identifiable assets	9,584.9				11,038.7				9,584.9	11,038.7	11,511.1
Goodwill	2,288.7				3,590.6				2,288.7	3,590.6	3,610.9	$ 3,586.8
Intangibles, net	2,244.2				2,404.4				2,244.2	2,404.4	2,615.8
Capital expenditures									217.1	365.9	308.3
Equity method investments	14.9				11.6				14.9	11.6	1.8
Consumer Packaging [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales									6,333.0	6,606.0	6,617.5
Segment income									323.7	388.1	445.1
Consumer Packaging [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Net sales									(121.3)	(81.8)	(80.5)
Consumer Packaging [Member] | Unaffiliated Customers [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales									6,211.7	6,524.2	6,537.0
Land and Development [Member]
Segment Reporting Information [Line Items]
Net sales									18.9	23.4
Depreciation, depletion and amortization											0.7
Identifiable assets					28.3					28.3	49.1
Land and Development [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales									18.9	23.4	142.4
Segment income									1.4	2.5	22.5
Land and Development [Member] | Unaffiliated Customers [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales									18.9	23.4	142.4
Corporate Segment [Member]
Segment Reporting Information [Line Items]
Depreciation, depletion and amortization									6.1	8.5	4.5
Identifiable assets	2,680.8				2,382.8				2,680.8	2,382.8	2,671.2
Capital expenditures									29.9	41.8	34.3
Equity method investments	0.4				0.4				0.4	0.4	0.4
Disposal Group, Held-for-sale, Not Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Identifiable assets	$ 7.0				$ 25.8				$ 7.0	$ 25.8	$ 59.5