Summary of Significant Accounting Policies - Asset Retirement Obligations (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2020	Sep. 30, 2019
Asset retirement obligations [Roll forward]
Asset retirement obligations, beginning of year	$ 337.6	$ 321.1
Liabilities incurred during the period	3.0	10.8
Liabilities settled during the period	(6.9)	(8.3)
Accretion	14.0	13.0
Revisions in estimated cash flows	192.4	1.0
Asset retirement obligations, end of year	540.1	337.6
Spire Missouri
Asset retirement obligations [Roll forward]
Asset retirement obligations, beginning of year	173.5	174.1
Liabilities incurred during the period	2.6	1.6
Liabilities settled during the period	(4.0)	(6.8)
Accretion	7.2	7.4
Revisions in estimated cash flows	(25.9)	(2.8)
Asset retirement obligations, end of year	153.4	173.5
Spire Alabama
Asset retirement obligations [Roll forward]
Asset retirement obligations, beginning of year	148.7	135.7
Liabilities incurred during the period	0.0	8.8
Liabilities settled during the period	(1.6)	(0.8)
Accretion	6.1	5.0
Revisions in estimated cash flows	221.1	0.0
Asset retirement obligations, end of year	$ 374.3	$ 148.7

The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

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- Definition

Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6392676&loc=d3e7480-110848

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- Definition

Amount of asset retirement obligations incurred during the period.

+ References

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- Definition

Amount of asset retirement obligations settled, or otherwise disposed of, during the period. This may include asset retirement obligations transferred to third parties associated with the sale of a long-lived asset.

+ References

+ Details

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- Definition

Amount of increase (decrease) in the asset retirement obligation from changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 35
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=6392603&loc=d3e7123-110846

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)(4)
-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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