Segment Information - Changes in Carrying Amount of Goodwill (Details) $ in Millions	9 Months Ended
	Jun. 30, 2019 USD ($)
Goodwill [Roll Forward]
Goodwill, Gross beginning of period	$ 5,620.4
Accumulated impairment losses, beginning of period	(42.8)
Goodwill, beginning of period	5,577.6
Goodwill acquired	1,751.9
Purchase price allocation adjustments	(1.1)
Translation and other adjustments	(6.5)
Goodwill, Gross end of period	7,364.7
Accumulated impairment losses, end of period	(42.8)
Goodwill, end of period	7,321.9
Corrugated Packaging [Member]
Goodwill [Roll Forward]
Goodwill, Gross beginning of period	1,966.8
Accumulated impairment losses, beginning of period	(0.1)
Goodwill, beginning of period	1,966.7
Goodwill acquired	1,745.1
Purchase price allocation adjustments	0.9
Translation and other adjustments	(6.9)
Goodwill, Gross end of period	3,705.9
Accumulated impairment losses, end of period	(0.1)
Goodwill, end of period	3,705.8
Consumer Packaging [Member]
Goodwill [Roll Forward]
Goodwill, Gross beginning of period	3,653.6
Accumulated impairment losses, beginning of period	(42.7)
Goodwill, beginning of period	3,610.9
Goodwill acquired	6.8
Purchase price allocation adjustments	(2.0)
Translation and other adjustments	0.4
Goodwill, Gross end of period	3,658.8
Accumulated impairment losses, end of period	(42.7)
Goodwill, end of period	$ 3,616.1