Segment Information - Additional Information (Details) $ in Millions	3 Months Ended			9 Months Ended
	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) segment	Jun. 30, 2019 USD ($)	Mar. 31, 2020 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment				3
Insurance proceeds received, net cash used for investing activities				$ 2.4	$ 16.5
Corrugated Packaging [Member] | Hurricane Michael [Member]
Segment Reporting Information [Line Items]
Insurance proceeds received		$ 32.3
Deferred insurance proceeds		2.8
Deferred insurance proceeds recognized in cost of goods sold						$ 2.8
Insurance proceeds from business interruption recoveries		11.7			45.0
Insurance proceeds for direct costs and property damage		20.6			65.0
Insurance proceeds received, net cash provided by operating activities				30.9	93.5
Insurance proceeds received, net cash used for investing activities				$ 1.4	16.5
Corrugated Packaging [Member] | Hurricane Michael [Member] | Cost of Goods Sold [Member]
Segment Reporting Information [Line Items]
Insurance proceeds received			$ 30.0		$ 110.0
Reduction of cost of goods sold		$ 29.5
Consumer Packaging [Member] | Cost of Goods Sold [Member]
Segment Reporting Information [Line Items]
Reduction of cost of goods sold	$ 1.0