PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Fair Value of Plan Assets (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Pension Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year		$ 506.6	$ 345.4
Actual return on plan assets		(7.2)	52.1
Employer contributions		40.1	23.6
Settlements		(71.1)	0.0
Gross benefits paid		(19.5)	(36.9)
Fair value of plan assets, end of year	$ 506.6	448.9	506.6
Funded status of plans, end of year	(185.8)	(203.4)	(185.8)
Pension Plans \| Missouri Gas Energy (MGE) \| Energen
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Energen divestiture		0.0	0.0
Pension Plans \| Alagasco
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Alagasco acquisition		0.0	122.4
Postretirement Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year		222.5	111.6
Actual return on plan assets		(2.0)	11.6
Employer contributions		17.9	19.1
Gross benefits paid		(15.1)	(7.3)
Fair value of plan assets, end of year	222.5	223.3	222.5
Funded status of plans, end of year	(36.0)	(15.9)	(36.0)
Postretirement Plans \| Energen
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Energen divestiture		0.0	0.0
Postretirement Plans \| Alagasco
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Alagasco acquisition		0.0	87.5
Laclede Gas \| Pension Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year		387.4	345.4
Actual return on plan assets		(3.0)	55.0
Employer contributions		30.1	23.6
Settlements		(58.2)	0.0
Gross benefits paid		(16.4)	(36.6)
Fair value of plan assets, end of year	387.4	339.9	387.4
Funded status of plans, end of year	(156.2)	(157.7)	(156.2)
Laclede Gas \| Pension Plans \| Missouri Gas Energy (MGE) \| Energen
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Energen divestiture		0.0	0.0
Laclede Gas \| Pension Plans \| Alagasco
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Alagasco acquisition		0.0	0.0
Laclede Gas \| Postretirement Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year		137.2	111.6
Actual return on plan assets		(0.4)	13.3
Employer contributions		17.9	19.1
Gross benefits paid		(11.1)	(6.8)
Fair value of plan assets, end of year	137.2	143.6	137.2
Funded status of plans, end of year	(60.7)	(48.3)	(60.7)
Laclede Gas \| Postretirement Plans \| Energen
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Energen divestiture		0.0	0.0
Laclede Gas \| Postretirement Plans \| Alagasco
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Alagasco acquisition		0.0	0.0
Alagasco \| Pension Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	219.5	119.2
Actual return on plan assets	7.8	(4.2)
Employer contributions	1.6	10.0
Settlements	0.0	(12.9)
Gross benefits paid	(33.8)	(3.1)
Fair value of plan assets, end of year	119.2	109.0	119.2
Funded status of plans, end of year	(29.6)	(45.7)	(29.6)
Alagasco \| Pension Plans \| Missouri Gas Energy (MGE) \| Energen
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Energen divestiture	(75.9)	0.0
Alagasco \| Pension Plans \| Alagasco
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Alagasco acquisition	0.0	0.0
Alagasco \| Postretirement Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	98.6	85.3
Actual return on plan assets	1.4	(1.6)
Employer contributions	0.3	0.0
Gross benefits paid	(4.0)	(4.0)
Fair value of plan assets, end of year	85.3	79.7	85.3
Funded status of plans, end of year	24.7	32.4	$ 24.7
Alagasco \| Postretirement Plans \| Energen
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Energen divestiture	(11.0)	0.0
Alagasco \| Postretirement Plans \| Alagasco
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Alagasco acquisition	$ 0.0	$ 0.0

Fair value of plan assets attributed to acquisitions.

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- Definition

The difference between fair value of plan assets at the end of the period and the fair value at the beginning of the period, adjusted for contributions and payments of benefits during the period, and after adjusting for taxes and other expenses, as applicable.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Actual Return on Plan Assets (Component of Net Periodic Postretirement Benefit Cost)

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-Name Accounting Standards Codification

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-Glossary Actual Return on Plan Assets (Component of Net Periodic Pension Cost)

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- Definition

The amount of payments made for which participants are entitled under a pension plan, including pension benefits, death benefits, and benefits due on termination of employment. Also includes payments made under a postretirement benefit plan, including prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. This item represents a periodic decrease to the plan obligations and a decrease to plan assets.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

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-Topic 715

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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

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- Definition

The increase in the fair value of plan assets from contributions made by the employer.

+ References

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- Definition

The amount of decrease in the plan assets attributed to the sale, liquidation, spin-off or other divestiture of a corporate division or subsidiary.

+ References

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- Definition

Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets.

+ References

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-Publisher FASB

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-SubTopic 30

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- Definition

The funded status is measured as the difference between the fair value of plan assets and the benefit obligation. Will normally be the same as the net Defined Benefit Plan, Amounts Recognized in Balance Sheet, Total.

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- Definition

The amount that relates to an irrevocable action that relieves the employer (or the plan) of primary responsibility for a benefit obligation and eliminates significant risks related to the obligation and the assets used to effect the settlement. Examples of transactions that constitute a settlement include (a) making lump-sum cash payments to plan participants in exchange for their rights to receive specified benefits and (b) purchasing nonparticipating annuity contracts.

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