Segment Information - Certain Operating Data for Segments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Sep. 30, 2021
Segment Reporting Information [Line Items]
Net sales	$ 5,519.7	$ 4,816.3	$ 15,854.0	$ 13,655.6
Adjusted EBITDA	1,038.3	830.4	2,606.5	2,177.2
Depreciation, depletion and amortization	(377.3)	(369.0)	(1,117.4)	(1,094.9)
Gain on sale of certain closed facilities			14.4	0.9
Multiemployer pension withdrawal income			3.3
Mineral rights impairment	(26.0)		(26.0)
Restructuring and other costs	(0.6)	(6.9)	(366.3)	(19.8)
Interest expense, net	(78.5)	(102.5)	(237.7)	(279.8)
Loss on extinguishment of debt			(8.2)	(1.1)
Other (expense) income, net	(7.2)	6.4	(0.7)	13.8
Other adjustments	(3.4)	(10.6)	(3.7)	(64.3)
Income before income taxes	512.5	328.4	797.4	676.1
Depreciation, depletion and amortization	377.3	369.0	1,117.4	1,094.9
Other adjustments	3.4	10.6	3.7	64.3
Assets	28,885.7		28,885.7		$ 29,254.3
Intangibles, net	3,031.3		3,031.3		3,318.8
Equity method investments	477.8		477.8		453.3
Capital expenditures			569.5	505.4
Assets Held for Sale [Member]
Segment Reporting Information [Line Items]
Assets	4.2		4.2		10.9
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	5,620.7	4,908.4	16,126.8	13,899.0
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Net sales	(101.0)	(92.1)	(272.8)	(243.4)
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Non-allocated expenses	(32.8)	(19.4)	(66.8)	(55.9)
Interest expense, net	(78.5)	(102.5)	(237.7)	(279.8)
Corrugated Packaging [Member]
Segment Reporting Information [Line Items]
Net sales	2,382.5	2,154.7	6,921.5	6,196.6
Depreciation, depletion and amortization	169.7	169.5	503.6	512.8
Other adjustments	0.8	1.9	(5.6)	13.4
Assets	11,461.2		11,461.2		11,526.0
Intangibles, net	677.6		677.6		765.9
Equity method investments	476.5		476.5		434.4
Capital expenditures			260.3	215.0
Corrugated Packaging [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	2,382.5	2,154.7	6,921.5	6,196.6
Adjusted EBITDA	385.2	363.9	1,002.8	1,032.6
Corrugated Packaging [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Net sales	(93.5)	(86.9)	(246.2)	(229.9)
Corrugated Packaging [Member] | Unaffiliated Customers [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	2,289.0	2,067.8	6,675.3	5,966.7
Consumer Packaging [Member]
Segment Reporting Information [Line Items]
Net sales	1,270.2	1,132.2	3,659.5	3,275.3
Depreciation, depletion and amortization	88.2	90.1	264.6	264.2
Other adjustments		1.5	7.7	11.2
Assets	6,725.0		6,725.0		6,750.5
Intangibles, net	1,583.7		1,583.7		1,719.2
Equity method investments	0.4		0.4		17.7
Capital expenditures			109.9	116.7
Consumer Packaging [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	1,270.2	1,132.2	3,659.5	3,275.3
Adjusted EBITDA	234.9	183.3	610.0	522.7
Consumer Packaging [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Net sales	(6.1)	(5.0)	(19.0)	(13.0)
Consumer Packaging [Member] | Unaffiliated Customers [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	1,264.1	1,127.2	3,640.5	3,262.3
Global Paper [Member]
Segment Reporting Information [Line Items]
Net sales	1,610.3	1,299.2	4,501.0	3,520.7
Depreciation, depletion and amortization	113.0	102.7	329.0	298.0
Other adjustments	2.6	0.9	1.6	3.3
Assets	7,305.1		7,305.1		7,525.7
Intangibles, net	628.9		628.9		677.7
Equity method investments	0.8		0.8		0.8
Capital expenditures			165.8	160.8
Global Paper [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	1,610.3	1,299.2	4,501.0	3,520.7
Adjusted EBITDA	399.0	265.2	940.0	576.5
Global Paper [Member] | Unaffiliated Customers [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	1,610.3	1,299.2	4,501.0	3,520.7
Distribution [Member]
Segment Reporting Information [Line Items]
Net sales	357.7	322.3	1,044.8	906.4
Depreciation, depletion and amortization	5.8	5.7	17.4	17.2
Other adjustments				0.6
Assets	813.9		813.9		796.2
Intangibles, net	141.1		141.1		156.0
Capital expenditures			2.9	0.5
Distribution [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	357.7	322.3	1,044.8	906.4
Adjusted EBITDA	19.2	18.0	53.7	45.4
Distribution [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Net sales	(1.4)	(0.2)	(7.6)	(0.5)
Distribution [Member] | Unaffiliated Customers [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	356.3	322.1	1,037.2	905.9
Corporate Segment [Member]
Segment Reporting Information [Line Items]
Depreciation, depletion and amortization	0.6	1.0	2.8	2.7
Other adjustments		$ 6.3		35.8
Assets	2,576.3		2,576.3		2,645.0
Equity method investments	$ 0.1		0.1		$ 0.4
Capital expenditures			$ 30.6	$ 12.4