Fair Value Measurements (Details) - USD ($) $ in Millions	3 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Quoted Prices In Active Markets for Identical Assets (Level 1)
Additional Fair Value Elements [Abstract]
Long-term debt. including current portion	$ 0.0	$ 0.0
Significant Other Oberservable Inputs (Level 2)
Additional Fair Value Elements [Abstract]
Long-term debt. including current portion	(6,949.0)	(5,251.4)
Significant Unobservable Inputs (Level 3)
Additional Fair Value Elements [Abstract]
Long-term debt. including current portion	0.0	0.0
Carrying Amount
Additional Fair Value Elements [Abstract]
Long-term debt. including current portion	(5,239.8)	(4,065.0)
Fair Value
Additional Fair Value Elements [Abstract]
Long-term debt. including current portion	(6,949.0)	(5,251.4)
Fair Value, Nonrecurring \| Significant Unobservable Inputs (Level 3)
Additional Fair Value Elements [Abstract]
Property, Plant, and Equipment, Fair Value Disclosure	41.5
Tangible Asset Impairment Charges	170.1
ARO Trust investments \| Quoted Prices In Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
ARO Trust investments	235.5	200.6
ARO Trust investments \| Significant Other Oberservable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
ARO Trust investments	0.0	0.0
ARO Trust investments \| Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
ARO Trust investments	0.0	0.0
ARO Trust investments \| Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
ARO Trust investments	235.5	200.6
ARO Trust investments \| Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
ARO Trust investments	$ 235.5	$ 200.6

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- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

No definition available.

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- References

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- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

No definition available.

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- Definition

Fair value portion of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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- Definition

The charge against earnings resulting from the aggregate write down of tangible assets from their carrying value to their fair value.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109226691&loc=d3e2921-110230

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109222650&loc=d3e1361-107760

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