Revenue Recognition - Schedule of Disaggregate Revenue by Geographical Market and Product Type (Segment) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Disaggregation Of Revenue [Line Items]
Net sales	$ 4,620.0	$ 4,017.0	$ 8,947.4	$ 7,911.0
Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	2,990.7		5,724.5
Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1,668.3		3,287.1
Land and Development [Member]
Disaggregation Of Revenue [Line Items]
Net sales	0.8		14.7
Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(39.8)	(38.3)	(78.9)	(76.7)
Intersegment Sales [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(19.4)	(22.0)	(37.4)	(41.4)
Intersegment Sales [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(20.4)	$ (16.3)	(41.5)	$ (35.3)
North America [Member]
Disaggregation Of Revenue [Line Items]
Net sales	4,134.6		7,973.4
North America [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	2,865.3		5,471.7
North America [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1,308.3		2,564.7
North America [Member] | Land and Development [Member]
Disaggregation Of Revenue [Line Items]
Net sales	0.8		14.7
North America [Member] | Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(39.8)		(77.7)
South America [Member]
Disaggregation Of Revenue [Line Items]
Net sales	127.5		255.8
South America [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	109.6		218.4
South America [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	17.9		37.4
Europe [Member]
Disaggregation Of Revenue [Line Items]
Net sales	269.6		530.1
Europe [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	269.6		530.1
Asia Pacific [Member]
Disaggregation Of Revenue [Line Items]
Net sales	88.3		188.1
Asia Pacific [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	15.8		34.4
Asia Pacific [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	$ 72.5		154.9
Asia Pacific [Member] | Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales			$ (1.2)