Acquisitions - Summary of Fair Values of Assets Acquired and Liabilities Assumed by Major Class of Assets and Liabilities and Measurement Period Adjustments (Details) - USD ($) $ in Millions		Jun. 30, 2020	Sep. 30, 2019	Jun. 30, 2019	Nov. 02, 2018
Business Acquisition [Line Items]
Goodwill		$ 7,270.5	$ 7,285.6
Liabilities assumed				$ 2,450.4
KapStone Acquisition [Member]
Business Acquisition [Line Items]
Cash and cash equivalents					$ 8.6
Current assets, excluding cash and cash equivalents					878.9
Property, plant and equipment, net					1,910.3
Goodwill					1,755.0
Intangible assets					1,336.1
Other long-term assets					27.9
Total assets acquired					5,916.8
Current portion of debt					33.3
Current liabilities					337.5
Long-term debt due after one year					1,333.4
Accrued pension and other long-term benefits					9.8
Deferred income taxes					609.7
Other long-term liabilities					118.4
Liabilities assumed					2,442.1
Net assets acquired					3,474.7
KapStone Acquisition [Member] | Measurement Period Adjustments [Member]
Business Acquisition [Line Items]
Current assets, excluding cash and cash equivalents	[1]				(30.2)
Property, plant and equipment, net	[1]				11.5
Goodwill	[1]				0.5
Intangible assets	[1]				30.3
Other long-term assets	[1]				(0.1)
Total assets acquired	[1]				12.0
Current liabilities	[1]				7.9
Accrued pension and other long-term benefits	[1]				2.8
Deferred income taxes	[1]				(1.4)
Other long-term liabilities	[1]				2.7
Liabilities assumed	[1]				12.0
KapStone Acquisition [Member] | As Adjusted [Member]
Business Acquisition [Line Items]
Cash and cash equivalents	[2]				8.6
Current assets, excluding cash and cash equivalents	[2]				848.7
Property, plant and equipment, net	[2]				1,921.8
Goodwill	[2]				1,755.5
Intangible assets	[2]				1,366.4
Other long-term assets	[2]				27.8
Total assets acquired	[2]				5,928.8
Current portion of debt	[2]				33.3
Current liabilities	[2]				345.4
Long-term debt due after one year	[2]				1,333.4
Accrued pension and other long-term benefits	[2]				12.6
Deferred income taxes	[2]				608.3
Other long-term liabilities	[2]				121.1
Liabilities assumed	[2]				2,454.1
Net assets acquired	[2]				$ 3,474.7

[1] ^{The measurement period adjustments recorded in fiscal 2019 and fiscal 2020 did not have a significant impact on our condensed consolidated statements of income in any period.}

[2] ^{The measurement period adjustments were primarily due to refinements to third party appraisals and carrying amounts of certain assets and liabilities, as well as adjustments to certain tax accounts based on, among other things, adjustments to deferred tax liabilities. The net impact of the measurement period adjustments to goodwill were essentially flat.}