PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Fair Value of Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2016	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	$ 448.9	$ 506.6
Actual return on plan assets	75.1	(7.2)
Employer contributions	26.6	40.1
EnergySouth acquisition	44.8	0.0
Settlement benefits paid	(16.6)	(71.1)	$ (22.1)
Regular benefits paid	(38.3)	(19.5)
Fair value of plan assets, end of year	540.5	448.9	506.6
Funded status of plans, end of year	(254.2)	(203.4)
Postretirement Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	223.3	222.5
Actual return on plan assets	19.9	(2.0)
Employer contributions	14.3	17.9
EnergySouth acquisition	4.0	0.0
Regular benefits paid	(15.1)	(15.1)
Fair value of plan assets, end of year	246.4	223.3	222.5
Funded status of plans, end of year	(12.8)	(15.9)
Laclede Gas \| Pension Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	339.9	387.4
Actual return on plan assets	64.4	(3.0)
Employer contributions	26.6	30.1
EnergySouth acquisition	0.0	0.0
Settlement benefits paid	0.0	(58.2)
Regular benefits paid	(35.2)	(16.4)
Fair value of plan assets, end of year	395.7	339.9	387.4
Funded status of plans, end of year	(164.3)	(157.7)
Laclede Gas \| Postretirement Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	143.6	137.2
Actual return on plan assets	13.8	(0.4)
Employer contributions	14.3	17.9
EnergySouth acquisition	0.0	0.0
Regular benefits paid	(12.0)	(11.1)
Fair value of plan assets, end of year	159.7	143.6	137.2
Funded status of plans, end of year	(48.2)	(48.3)
Alagasco \| Pension Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	109.0	119.2
Actual return on plan assets	10.7	(4.2)
Employer contributions	0.0	10.0
EnergySouth acquisition	0.0	0.0
Settlement benefits paid	(16.6)	(12.9)
Regular benefits paid	(3.1)	(3.1)
Fair value of plan assets, end of year	100.0	109.0	119.2
Funded status of plans, end of year	(74.3)	(45.7)
Alagasco \| Postretirement Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	79.7	85.3
Actual return on plan assets	6.2	(1.6)
Employer contributions	0.0	0.0
EnergySouth acquisition	0.0	0.0
Regular benefits paid	(3.1)	(4.0)
Fair value of plan assets, end of year	82.8	79.7	$ 85.3
Funded status of plans, end of year	$ 37.4	$ 32.4

Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Remeasurement due to Settlement

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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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- 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.

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- Definition

The amount of increase in the plan assets attributed to a business combination.

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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

Amount of contributions made by the employer to defined benefit plans.

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- Definition

Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees.

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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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