Retirement Benefits - Schedule of Amounts Included in Accumulated Other Comprehensive Loss, Net of Tax (Details) - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Prior service (credit) cost	$ (30.7)	$ 3.3
Net actuarial loss	718.7	1,262.1
Total	688.0	1,265.4
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Prior service (credit) cost	4.1	0.0
Net actuarial loss	2.0	5.8
Total	$ 6.1	$ 5.8

Amount, after tax, of accumulated other comprehensive (income) loss for defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
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- Definition

Amount, after tax, of accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 14A
-URI http://asc.fasb.org/extlink&oid=124509347&loc=SL7669686-108580

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

Amount, after tax, of accumulated other comprehensive (income) loss for cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation of defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (j)
-URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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-Section 45
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-URI http://asc.fasb.org/extlink&oid=124509347&loc=SL7669686-108580

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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