SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Asset Retirement Obligations (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Sep. 30, 2014	Sep. 30, 2013
Asset retirement obligations [Roll forward]
Asset retirement obligations, beginning of year	$ 74.6	$ 40.4
Liabilities incurred during the period	0.5	0.8
Liabilities settled during the period	(1.5)	(1.1)
Accretion	3.7	2.3
Revisions in estimated cash flows	(5.8)	0
Asset retirement obligations, end of year	99.2	74.6
Laclede Gas
Asset retirement obligations [Roll forward]
Asset retirement obligations, beginning of year	74.3	40.1
Liabilities incurred during the period	0.5	0.8
Liabilities settled during the period	(1.5)	(1.1)
Accretion	3.7	2.3
Revisions in estimated cash flows	(5.8)	0
Asset retirement obligations, end of year	71.2	74.3
Missouri Gas Energy (MGE)
Asset retirement obligations [Roll forward]
Addition of asset retirement obligation	0	32.2
Missouri Gas Energy (MGE) \| Laclede Gas
Asset retirement obligations [Roll forward]
Addition of asset retirement obligation	0	32.2
Alagasco
Asset retirement obligations [Roll forward]
Addition of asset retirement obligation	27.7	0
Alagasco \| Laclede Gas
Asset retirement obligations [Roll forward]
Addition of asset retirement obligation	$ 0	$ 0

The amount of increase in the ARO attributed to business combinations.

No definition available.

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Asset Retirement Obligation

-URI http://asc.fasb.org/extlink&oid=6505190

+ Details

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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://www.xbrl.org/2003/role/presentationRef

-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

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-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

+ Details

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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 the Increase or Decrease in the amount of the asset retirement obligation during the current period for changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation.

+ References

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