Retirement and Post-Retirement Employee Benefit Plans - Asset Allocation Postretirement Benefit Plan (Details)	Sep. 30, 2021	Sep. 30, 2020
Postretirement Plans \| Diversified investment funds
Defined Benefit Plan Disclosure [Line Items]
Asset allocation	97.90%	97.40%
Postretirement Plans \| Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Asset allocation	2.10%	2.60%
Defined Benefit Plan \| Domestic equities
Defined Benefit Plan Disclosure [Line Items]
Asset allocation	44.50%	45.30%
Defined Benefit Plan \| Domestic equities \| Minimum
Defined Benefit Plan Disclosure [Line Items]
Targeted Allocation Range	35.00%
Defined Benefit Plan \| Domestic equities \| Maximum
Defined Benefit Plan Disclosure [Line Items]
Targeted Allocation Range	55.00%

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Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=123450688&loc=d3e4179-114921

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(i)
-URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920

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Percentage of actual investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(i)
-URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920

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