Revenue Recognition - Schedule of Disaggregates Revenue by Geographical Market and Product Type (Segment) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Disaggregation Of Revenue [Line Items]
Net sales	$ 4,447.3	$ 4,620.0	$ 8,871.0	$ 8,947.4
Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	2,882.5	2,990.7	5,792.0	5,724.5
Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1,616.3	1,668.3	3,153.2	3,287.1
Land and Development [Member]
Disaggregation Of Revenue [Line Items]
Net sales		0.8	18.9	14.7
Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(51.5)	(39.8)	(93.1)	(78.9)
Intersegment Sales [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(20.7)	(19.4)	(39.0)	(37.4)
Intersegment Sales [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(30.8)	(20.4)	(54.1)	(41.5)
North America [Member]
Disaggregation Of Revenue [Line Items]
Net sales	3,995.9	4,134.6	7,961.0	7,973.4
North America [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	2,765.7	2,865.3	5,554.6	5,471.7
North America [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1,281.6	1,308.3	2,480.4	2,564.7
North America [Member] | Land and Development [Member]
Disaggregation Of Revenue [Line Items]
Net sales		0.8	18.9	14.7
North America [Member] | Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(51.4)	(39.8)	(92.9)	(77.7)
South America [Member]
Disaggregation Of Revenue [Line Items]
Net sales	119.5	127.5	247.1	255.8
South America [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	100.7	109.6	208.3	218.4
South America [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	18.8	17.9	38.8	37.4
Europe [Member]
Disaggregation Of Revenue [Line Items]
Net sales	255.9	269.6	502.3	530.1
Europe [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	2.5		4.3
Europe [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	253.4	269.6	498.0	530.1
Asia Pacific [Member]
Disaggregation Of Revenue [Line Items]
Net sales	76.0	88.3	160.6	188.1
Asia Pacific [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	13.6	15.8	24.8	34.4
Asia Pacific [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	62.5	$ 72.5	136.0	154.9
Asia Pacific [Member] | Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	$ (0.1)		$ (0.2)	$ (1.2)