Difference between amount at which an investment accounted for under the equity method of accounting is carried (reported) on the balance sheet and amount of underlying equity in net assets the reporting Entity has in the investee.

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The net amount (to the extent disclosed within this portion of the income statement) by which an insurance settlement exceeds incremental costs incurred from the event causing an interruption of business, plus the insurance award for earnings lost from the event, such as a natural catastrophe, explosion or fire.

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Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

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Amount of cash inflow for proceeds from settlement of insurance claim, classified as investing activities. Excludes insurance settlement classified as operating activities.

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Amount of cash inflow for proceeds from settlement of insurance claim, classified as operating activities. Excludes insurance settlement classified as investing activities.

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Amount of insurance proceeds for an event or transaction that is unusual in nature or infrequent in occurrence, or both.

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Direct costs and property damage insurance recovery.

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Pre-tax inventory step-up.

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Segment Information - Additional Information (Details) $ in Millions	3 Months Ended			12 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2020 USD ($) segment	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments \| segment				3
Insurance proceeds received, net cash used for investing activities				$ 6.5	$ 25.5	$ 7.9
Goodwill impairment loss	$ 1,333.2			1,333.2
Hurricane Michael [Member]
Segment Reporting Information [Line Items]
Insurance proceeds received		$ 70.0	$ 20.0
Corrugated Packaging [Member]
Segment Reporting Information [Line Items]
Pre-Tax Inventory Step-Up					24.7	$ 1.0
Corrugated Packaging [Member] \| Grupo Gondi Investment [Member]
Segment Reporting Information [Line Items]
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity	101.7	121.4		101.7	121.4
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity, Amortizable Portion	$ 41.9	$ 53.1		$ 41.9	53.1
Corrugated Packaging [Member] \| Grupo Gondi Investment [Member] \| Minimum [Member]
Segment Reporting Information [Line Items]
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity, Lives of Underlying Assets				10 years
Corrugated Packaging [Member] \| Grupo Gondi Investment [Member] \| Maximum [Member]
Segment Reporting Information [Line Items]
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity, Lives of Underlying Assets				15 years
Corrugated Packaging [Member] \| Hurricane Michael [Member]
Segment Reporting Information [Line Items]
Insurance proceeds from business interruption recoveries				$ 11.7	55.3
Insurance proceeds for direct costs and property damage				20.6	124.7
Insurance proceeds received, net cash provided by operating activities				30.9	154.5
Insurance proceeds received, net cash used for investing activities				1.4	25.5
Corrugated Packaging [Member] \| Hurricane Michael [Member] \| Cost of Goods Sold [Member]
Segment Reporting Information [Line Items]
Insurance proceeds received				32.3	$ 180.0
Consumer Packaging [Member]
Segment Reporting Information [Line Items]
Goodwill impairment loss				$ 1,333.2