PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Fair Value of Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Settlement benefits paid		$ (62.2)
Pension Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	$ 531.6	540.5
Actual return on plan assets	11.2	38.0
Employer contributions	58.9	41.3
Divestitures	0.0	0.0
Settlement benefits paid	(82.8)	(62.2)
Regular benefits paid	(19.7)	(26.0)
Fair value of plan assets, end of year	499.2	531.6	$ 540.5
Funded status of plans, end of year	(165.4)	(217.2)
Postretirement Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	265.5	246.4
Actual return on plan assets	21.5	26.2
Employer contributions	7.6	10.4
Regular benefits paid	(11.1)	(17.5)
Fair value of plan assets, end of year	283.5	265.5	246.4
Funded status of plans, end of year	75.4	27.0
Spire Missouri \| Pension Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	385.9	395.7
Actual return on plan assets	6.4	25.1
Employer contributions	36.9	29.4
Divestitures	(0.3)	0.0
Settlement benefits paid	(65.5)	(43.5)
Regular benefits paid	(14.3)	(20.8)
Fair value of plan assets, end of year	349.1	385.9	395.7
Funded status of plans, end of year	(124.2)	(153.7)
Spire Missouri \| Postretirement Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	174.9	159.7
Actual return on plan assets	15.6	16.8
Employer contributions	7.6	10.4
Regular benefits paid	(8.6)	(12.0)
Fair value of plan assets, end of year	189.5	174.9	159.7
Funded status of plans, end of year	30.7	(17.6)
Spire Alabama
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, end of year	8.8
Spire Alabama \| Pension Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	97.9	100.0
Actual return on plan assets	3.3	7.7
Employer contributions	20.1	11.9
Divestitures	0.0	0.0
Settlement benefits paid	(17.3)	(18.7)	(16.6)
Regular benefits paid	(2.7)	(3.0)
Fair value of plan assets, end of year	101.3	97.9	100.0
Funded status of plans, end of year	(35.0)	(50.3)
Spire Alabama \| Postretirement Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	86.4	82.8
Actual return on plan assets	5.6	8.9
Employer contributions	0.0	0.0
Regular benefits paid	(2.4)	(5.3)
Fair value of plan assets, end of year	89.6	86.4	$ 82.8
Funded status of plans, end of year	$ 46.5	$ 45.8

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

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Amount of decrease in plan assets of defined benefit plan from disposal or classification as held-for-sale.

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Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee.

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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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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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