Pension Plans and Other Postretirement Benefits - Fair Value of Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Pension Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	$ 499.2	$ 531.6
Actual return on plan assets	50.1	11.2
Employer contributions	27.8	58.9
Settlement benefits paid		(82.8)	$ (62.2)
Regular benefits paid	(55.3)	(19.7)
Fair value of plan assets, end of year	521.8	499.2	531.6
Funded status of plans, end of year	(229.6)	(165.4)
Pension Plans \| Spire Missouri
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	349.1	385.9
Actual return on plan assets	42.4	6.4
Employer contributions	27.1	36.9
Divestitures		(0.3)
Settlement benefits paid		(65.5)	(43.5)
Regular benefits paid	(39.4)	(14.3)
Fair value of plan assets, end of year	379.2	349.1	385.9
Funded status of plans, end of year	(159.2)	(124.2)
Pension Plans \| Spire Alabama
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	101.3	97.9
Actual return on plan assets	4.7	3.3
Employer contributions	0.5	20.1
Settlement benefits paid		(17.3)	(18.7)
Regular benefits paid	(13.2)	(2.7)
Fair value of plan assets, end of year	93.3	101.3	97.9
Funded status of plans, end of year	(59.2)	(35.0)
Other Postretirement Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	283.5	265.5
Actual return on plan assets	9.7	21.5
Employer contributions	0.6	7.6
Regular benefits paid	(14.0)	(11.1)
Fair value of plan assets, end of year	279.8	283.5	265.5
Funded status of plans, end of year	82.5	75.4
Other Postretirement Plans \| Spire Missouri
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	189.5	174.9
Actual return on plan assets	9.0	15.6
Employer contributions	0.6	7.6
Regular benefits paid	(10.2)	(8.6)
Fair value of plan assets, end of year	188.9	189.5	174.9
Funded status of plans, end of year	41.0	30.7
Other Postretirement Plans \| Spire Alabama
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	89.6	86.4
Actual return on plan assets	0.5	5.6
Regular benefits paid	(3.7)	(2.4)
Fair value of plan assets, end of year	86.4	89.6	$ 86.4
Funded status of plans, end of year	$ 43.0	$ 46.5

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

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

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- Definition

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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- 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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