Revenue Recognition - Schedule of Disaggregate Revenue by Geographical Market and Product Type (Segment) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation Of Revenue [Line Items]
Net sales	$ 4,690.0	$ 4,137.5	$ 13,637.4	$ 12,048.5
Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	3,072.8		8,797.3
Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1,650.1		4,937.2
Land and Development [Member]
Disaggregation Of Revenue [Line Items]
Net sales	8.6		23.3
Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(41.5)	(41.5)	(120.4)	(118.2)
Intersegment Sales [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(19.7)	(21.6)	(57.1)	(63.0)
Intersegment Sales [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(21.8)	$ (19.9)	(63.3)	$ (55.2)
North America [Member]
Disaggregation Of Revenue [Line Items]
Net sales	4,227.9		12,201.3
North America [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	2,954.6		8,426.3
North America [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1,306.0		3,870.7
North America [Member] | Land and Development [Member]
Disaggregation Of Revenue [Line Items]
Net sales	8.6		23.3
North America [Member] | Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(41.3)		(119.0)
South America [Member]
Disaggregation Of Revenue [Line Items]
Net sales	117.6		373.4
South America [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	102.8		321.2
South America [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	14.8		52.2
Europe [Member]
Disaggregation Of Revenue [Line Items]
Net sales	259.9		790.0
Europe [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	0.7		0.7
Europe [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	259.3		789.4
Europe [Member] | Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(0.1)		(0.1)
Asia Pacific [Member]
Disaggregation Of Revenue [Line Items]
Net sales	84.6		272.7
Asia Pacific [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	14.7		49.1
Asia Pacific [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	70.0		224.9
Asia Pacific [Member] | Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	$ (0.1)		$ (1.3)