Revenue Recognition - Schedule of Disaggregates Revenue by Geographical Market and Product Type (Segment) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation Of Revenue [Line Items]
Net sales	$ 4,236.3	$ 4,690.0	$ 13,107.3	$ 13,637.4
Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	2,728.8	3,072.8	8,520.8	8,797.3
Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1,552.6	1,650.1	4,705.8	4,937.2
Land and Development [Member]
Disaggregation Of Revenue [Line Items]
Net sales		8.6	18.9	23.3
Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(45.1)	(41.5)	(138.2)	(120.4)
Intersegment Sales [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(15.3)	(19.7)	(54.3)	(57.1)
Intersegment Sales [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(29.8)	(21.8)	(83.9)	(63.3)
North America [Member]
Disaggregation Of Revenue [Line Items]
Net sales	3,826.0	4,227.9	11,787.0	12,201.3
North America [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	2,632.4	2,954.6	8,187.0	8,426.3
North America [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1,238.4	1,306.0	3,718.8	3,870.7
North America [Member] | Land and Development [Member]
Disaggregation Of Revenue [Line Items]
Net sales		8.6	18.9	23.3
North America [Member] | Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(44.8)	(41.3)	(137.7)	(119.0)
South America [Member]
Disaggregation Of Revenue [Line Items]
Net sales	101.9	117.6	349.0	373.4
South America [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	87.8	102.8	296.1	321.2
South America [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	14.1	14.8	52.9	52.2
Europe [Member]
Disaggregation Of Revenue [Line Items]
Net sales	234.2	259.9	736.5	790.0
Europe [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	0.6	0.7	4.9	0.7
Europe [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	233.8	259.3	731.8	789.4
Europe [Member] | Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	(0.2)	(0.1)	(0.2)	(0.1)
Asia Pacific [Member]
Disaggregation Of Revenue [Line Items]
Net sales	74.2	84.6	234.8	272.7
Asia Pacific [Member] | Corrugated Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	8.0	14.7	32.8	49.1
Asia Pacific [Member] | Consumer Packaging [Member]
Disaggregation Of Revenue [Line Items]
Net sales	66.3	70.0	202.3	224.9
Asia Pacific [Member] | Intersegment Sales [Member]
Disaggregation Of Revenue [Line Items]
Net sales	$ (0.1)	$ (0.1)	$ (0.3)	$ (1.3)