Revenue Recognition - Schedule of Disaggregates Revenue by Geographical Market and Product Type (Segment) (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Disaggregation Of Revenue [Line Items]
Net sales	$ 4,423.7	$ 4,327.4
Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	2,909.5	2,733.8
Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1,536.9	1,618.8
Land and Development [Member]
Disaggregation Of Revenue [Line Items]
Net sales	18.9	13.9
Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(41.6)	(39.1)
Intersegment Sales [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(18.3)	(18.0)
Intersegment Sales [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(23.3)	(21.1)
North America [Member]
Disaggregation Of Revenue [Line Items]
Net sales	3,965.1	3,838.8
North America [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	2,788.9	2,606.4
North America [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1,198.8	1,256.4
North America [Member] | Land and Development [Member]
Disaggregation Of Revenue [Line Items]
Net sales	18.9	13.9
North America [Member] | Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(41.5)	(37.9)
South America [Member]
Disaggregation Of Revenue [Line Items]
Net sales	127.6	128.3
South America [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	107.6	108.8
South America [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	20.0	19.5
Europe [Member]
Disaggregation Of Revenue [Line Items]
Net sales	246.4	260.5
Europe [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1.8
Europe [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	244.6	260.5
Asia Pacific [Member]
Disaggregation Of Revenue [Line Items]
Net sales	84.6	99.8
Asia Pacific [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	11.2	18.6
Asia Pacific [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	73.5	82.4
Asia Pacific [Member] | Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	$ (0.1)	$ (1.2)