PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Assumed Medical Cost Trend Rates and Effect of 1% Changed (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Sep. 30, 2014	Sep. 30, 2013
Schedule of Effect of One Percentage Point Change [Line Items]
Rate to which the medical cost trend rate is assumed to decline (the ultimate medical cost trend rate)	5.00%	5.00%
Year the rate reaches the ultimate trend	2020	2020
Assumed 1% change in the assumed medical cost trend rate [Abstract]
Effect of 1% increase on net periodic postretirement benefit cost	$ 1.4
Effect of 1% decrease on net periodic postretirement benefit cost	(1.3)
Effect of 1% increase on accumulated postretirement benefit obligation	7.1
Effect of 1% decrease on accumulated postretirement benefit obligation	(6.7)
Missouri Gas Energy (MGE)
Schedule of Effect of One Percentage Point Change [Line Items]
Medical cost trend assumed for next year	7.50%	7.50%
Laclede Gas
Schedule of Effect of One Percentage Point Change [Line Items]
Medical cost trend assumed for next year	7.50%	7.50%
Assumed 1% change in the assumed medical cost trend rate [Abstract]
Effect of 1% increase on net periodic postretirement benefit cost	1.4
Effect of 1% decrease on net periodic postretirement benefit cost	(1.3)
Effect of 1% increase on accumulated postretirement benefit obligation	6.4
Effect of 1% decrease on accumulated postretirement benefit obligation	$ (5.9)
Alagasco
Schedule of Effect of One Percentage Point Change [Line Items]
Medical cost trend assumed for next year	7.25%	0.00%

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- Definition

Amount of effect of a one-percentage-point decrease in the assumed health care cost trend rates on the accumulated postretirement benefit obligation for health care benefits.

+ References

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Amount of effect of a one-percentage-point decrease in the assumed health care cost trend rates on the aggregate of the service and interest cost components of net periodic postretirement health care benefit costs.

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- Definition

The effect of a one-percentage-point increase in the assumed health care cost trend rates on the accumulated postretirement benefit obligation for health care benefits.

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- Definition

The effect of a one-percentage-point increase in the assumed health care cost trend rates on the aggregate of the service and interest cost components of net periodic postretirement health care benefit costs.

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- Definition

The assumed health care cost trend rate for the next year used to measure the expected cost of benefits covered by the plan (gross eligible charges). This is based upon the annual rate of change in the cost of health care benefits currently provided by the postretirement benefit plan, due to factors other than changes in the composition of the plan population by age and dependency status.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (l)

-URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Health Care Cost Trend Rate

-URI http://asc.fasb.org/extlink&oid=6514820

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- Definition

The ultimate trend rate for health care costs.

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- Definition

Year the ultimate health care cost trend rate is expected to be reached, in CCYY format.

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