PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Fair Value of Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Pension Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	$ 540.5	$ 448.9
Actual return on plan assets	38.0	75.1
Employer contributions	41.3	26.6
Spire EnergySouth acquisition	0.0	44.8
Settlement benefits paid	(62.2)	(16.6)	$ (71.1)
Regular benefits paid	(26.0)	(38.3)
Fair value of plan assets, end of year	531.6	540.5	448.9
Funded status of plans, end of year	(217.2)	(254.2)
Postretirement Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	246.4	223.3
Actual return on plan assets	26.2	19.9
Employer contributions	10.4	14.3
Spire EnergySouth acquisition	0.0	4.0
Regular benefits paid	(17.5)	(15.1)
Fair value of plan assets, end of year	265.5	246.4	223.3
Funded status of plans, end of year	27.0	(12.8)
Spire Missouri \| Pension Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	395.7	339.9
Actual return on plan assets	25.1	64.4
Employer contributions	29.4	26.6
Spire EnergySouth acquisition	0.0	0.0
Settlement benefits paid	(43.5)	0.0	(58.2)
Regular benefits paid	(20.8)	(35.2)
Fair value of plan assets, end of year	385.9	395.7	339.9
Funded status of plans, end of year	(153.7)	(164.3)
Spire Missouri \| Postretirement Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	159.7	143.6
Actual return on plan assets	16.8	13.8
Employer contributions	10.4	14.3
Spire EnergySouth acquisition	0.0	0.0
Regular benefits paid	(12.0)	(12.0)
Fair value of plan assets, end of year	174.9	159.7	143.6
Funded status of plans, end of year	(17.6)	(48.2)
Spire Alabama \| Pension Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	100.0	109.0
Actual return on plan assets	7.7	10.7
Employer contributions	11.9	0.0
Spire EnergySouth acquisition	0.0	0.0
Settlement benefits paid	(18.7)	(16.6)	(12.9)
Regular benefits paid	(3.0)	(3.1)
Fair value of plan assets, end of year	97.9	100.0	109.0
Funded status of plans, end of year	(50.3)	(74.3)
Spire Alabama \| Postretirement Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	82.8	79.7
Actual return on plan assets	8.9	6.2
Employer contributions	0.0	0.0
Spire EnergySouth acquisition	0.0	0.0
Regular benefits paid	(5.3)	(3.1)
Fair value of plan assets, end of year	86.4	82.8	$ 79.7
Funded status of plans, end of year	$ 45.8	$ 37.4

Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses.

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- Definition

Amount of (increase) decrease to benefit obligation of defined benefit plan from remeasurement due to settlement. Excludes decrease to benefit obligation for settlement payment.

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- Definition

Amount of increase in plan assets of defined benefit plan from 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.

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- Definition

Amount of contribution received by defined benefit plan from employer which increases plan assets.

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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

Amount of funded (unfunded) status of defined benefit plan, measured as difference between fair value of plan assets and benefit obligation. Includes, but is not limited to, overfunded (underfunded) status.

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- Definition

Amount of payment to participant under defined benefit plan which decreases plan assets. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services.

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