Segment Information - Certain Operating Data for Segments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Mar. 31, 2023	Mar. 31, 2022	Sep. 30, 2022
Segment Reporting Information [Line Items]
Net sales	$ 5,277.6	$ 5,382.1	$ 10,200.7	$ 10,334.3
Adjusted EBITDA	822.5	871.1	1,503.3	1,568.2
Depreciation, depletion and amortization	(395.8)	(373.6)	(769.0)	(740.1)
Gain on sale of certain closed facilities	8.9		9.8	14.4
Multiemployer pension withdrawal income				3.3
Restructuring and other costs	(444.7)	(363.4)	(477.7)	(365.7)
Goodwill impairment	(1,893.0)		(1,893.0)
Non-allocated expenses	(33.9)	(17.2)	(62.6)	(34.0)
Interest expense, net	(108.4)	(72.5)	(205.7)	(159.2)
Loss on extinguishment of debt		(8.2)		(8.2)
Other (expense) income, net	(17.8)	6.3	7.4	6.5
Other adjustments	(59.4)		(179.0)	(0.3)
(Loss) income before income taxes	(2,121.6)	42.5	(2,066.5)	284.9
Depreciation, depletion and amortization	395.8	373.6	769.0	740.1
Other adjustments	59.4		179.0	0.3
Equity in income (loss) of unconsolidated entities	4.5	20.6	(31.5)	39.0
Assets	28,210.4		28,210.4		$ 28,405.5
Intangibles, net	2,759.1		2,759.1		2,920.6
Equity method investments	80.5		80.5		480.4
Capital expenditures			563.7	354.1
Assets Held for Sale [Member]
Segment Reporting Information [Line Items]
Assets	169.2		169.2		34.4
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	5,368.0	5,470.0	10,365.5	10,506.1
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Net sales	(90.4)	(87.9)	(164.8)	(171.8)
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Interest expense, net	(108.4)	(72.5)	(205.7)	(159.2)
Corrugated Packaging [Member]
Segment Reporting Information [Line Items]
Net sales	2,627.4	2,319.0	4,964.8	4,539.0
Goodwill impairment	(514.3)
Depreciation, depletion and amortization	211.2	166.9	403.4	333.9
Other adjustments	4.7	(6.4)	54.5	(6.4)
Equity in income (loss) of unconsolidated entities	4.0	20.6	(31.8)	35.6
Assets	13,066.0		13,066.0		11,382.5
Intangibles, net	608.2		608.2		648.4
Equity method investments	78.6		78.6		479.3
Capital expenditures			241.7	156.7
Corrugated Packaging [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	2,627.4	2,319.0	4,964.8	4,539.0
Adjusted EBITDA	407.5	328.7	736.9	617.6
Corrugated Packaging [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Net sales	(81.6)	(78.5)	(150.0)	(152.7)
Corrugated Packaging [Member] | Unaffiliated Customers [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	2,545.8	2,240.5	4,814.8	4,386.3
Consumer Packaging [Member]
Segment Reporting Information [Line Items]
Net sales	1,265.1	1,250.6	2,480.1	2,389.3
Depreciation, depletion and amortization	85.5	90.1	169.6	176.4
Other adjustments	28.0	7.5	59.6	7.7
Equity in income (loss) of unconsolidated entities				3.4
Assets	6,647.0		6,647.0		6,704.5
Intangibles, net	1,460.9		1,460.9		1,523.5
Equity method investments	0.6		0.6		0.5
Capital expenditures			125.5	67.9
Consumer Packaging [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	1,265.1	1,250.6	2,480.1	2,389.3
Adjusted EBITDA	218.6	205.8	401.9	375.1
Consumer Packaging [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Net sales	(7.5)	(6.9)	(12.2)	(12.9)
Consumer Packaging [Member] | Unaffiliated Customers [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	1,257.6	1,243.7	2,467.9	2,376.4
Global Paper [Member]
Segment Reporting Information [Line Items]
Net sales	1,168.2	1,538.1	2,291.8	2,890.7
Goodwill impairment	(1,378.7)
Depreciation, depletion and amortization	91.2	109.8	180.3	216.0
Other adjustments	9.1	(1.1)	26.6	(1.0)
Equity in income (loss) of unconsolidated entities	0.5		0.3
Assets	5,151.1		5,151.1		7,039.2
Intangibles, net	563.8		563.8		612.6
Equity method investments	1.2		1.2		0.5
Capital expenditures			144.1	108.8
Global Paper [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	1,168.2	1,538.1	2,291.8	2,890.7
Adjusted EBITDA	187.1	308.6	344.4	541.0
Global Paper [Member] | Unaffiliated Customers [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	1,168.2	1,538.1	2,291.8	2,890.7
Distribution [Member]
Segment Reporting Information [Line Items]
Net sales	307.3	362.3	628.8	687.1
Depreciation, depletion and amortization	6.9	5.8	13.8	11.6
Assets	813.5		813.5		863.0
Intangibles, net	126.2		126.2		136.1
Capital expenditures			7.6	2.5
Distribution [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	307.3	362.3	628.8	687.1
Adjusted EBITDA	9.3	28.0	20.1	34.5
Distribution [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Net sales	(1.3)	(2.5)	(2.6)	(6.2)
Distribution [Member] | Unaffiliated Customers [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Net sales	306.0	359.8	626.2	680.9
Corporate Segment [Member]
Segment Reporting Information [Line Items]
Depreciation, depletion and amortization	1.0	$ 1.0	1.9	2.2
Other adjustments	17.6		38.3
Assets	2,363.6		2,363.6		2,381.9
Equity method investments	$ 0.1		0.1		$ 0.1
Capital expenditures			$ 44.8	$ 18.2