PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Benefit Obligation Rollforward (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Settlement benefits paid		$ (62.2)
Pension Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	$ 748.8	794.7
Service cost	20.2	20.5	$ 15.3
Interest cost	27.4	27.9	28.0
Actuarial (gain) loss	34.9	0.9
Plan amendments	(2.0)	(20.7)
Plan mergers	0.0	0.0
Settlement loss	7.6	14.6
Special termination benefits	0.0	0.9
Settlement benefits paid	(82.8)	(62.2)
Regular benefits paid	(19.7)	(26.0)
Benefit obligation, end of year	664.6	748.8	794.7
Accumulated benefit obligation, end of year	636.0	701.4
Postretirement Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	238.5	259.2
Service cost	9.4	11.0	10.9
Interest cost	8.6	8.6	10.2
Actuarial (gain) loss	(37.5)	(22.1)
Plan amendments	0.0	(1.4)
Curtailments	0.0	0.4
Retiree drug subsidy program	0.2	0.3
Regular benefits paid	(11.1)	(17.5)
Benefit obligation, end of year	208.1	238.5	259.2
Spire Missouri \| Pension Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	539.6	560.0
Service cost	12.7	12.7	10.0
Interest cost	19.5	19.5	21.7
Actuarial (gain) loss	26.0	0.5
Plan amendments	0.0	0.0
Plan mergers	(0.3)	0.0
Settlement loss	7.6	12.2
Special termination benefits	0.0	0.0
Settlement benefits paid	(65.5)	(43.5)
Regular benefits paid	(14.3)	(20.8)
Benefit obligation, end of year	473.3	539.6	560.0
Accumulated benefit obligation, end of year	446.5	500.4
Spire Missouri \| Postretirement Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	192.5	207.9
Service cost	9.0	10.4	10.6
Interest cost	6.9	6.8	8.1
Actuarial (gain) loss	(41.1)	(20.9)
Plan amendments	0.0	0.0
Curtailments	0.0	0.0
Retiree drug subsidy program	0.1	0.3
Regular benefits paid	(8.6)	(12.0)
Benefit obligation, end of year	158.8	192.5	207.9
Spire Alabama \| Pension Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	148.2	174.3
Service cost	6.4	6.2	5.3
Interest cost	5.5	6.1	6.3
Actuarial (gain) loss	3.8	(1.6)
Plan amendments	0.0	(20.7)
Plan mergers	0.0	0.0
Settlement loss	0.0	2.4
Special termination benefits	0.0	0.0
Settlement benefits paid	(17.3)	(18.7)	(16.6)
Regular benefits paid	(2.7)	(3.0)
Benefit obligation, end of year	136.3	148.2	174.3
Accumulated benefit obligation, end of year	134.5	142.8
Spire Alabama \| Postretirement Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	40.6	45.4
Service cost	0.2	0.3	0.3
Interest cost	1.5	1.6	2.1
Actuarial (gain) loss	3.1	0.0
Plan amendments	0.0	(1.4)
Curtailments	0.0	0.0
Retiree drug subsidy program	0.1	0.0
Regular benefits paid	(2.4)	(5.3)
Benefit obligation, end of year	$ 43.1	$ 40.6	$ 45.4

Amount of actuarial present value of benefits attributed to employee service rendered, excluding assumptions about future compensation level.

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Amount of gain (loss) from change in actuarial assumptions which (increases) decreases benefit obligation of defined benefit plan. Assumptions include, but are not limited to, interest, mortality, employee turnover, salary, and temporary deviation from substantive plan.

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Amount of actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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Amount of payment to participant of defined benefit plan which decreases benefit obligation. 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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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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Amount of payment, which decreases benefit obligation of defined benefit plan, for irrevocable action relieving primary responsibility for benefit obligation and eliminating risk for obligation and assets used to effect settlement. Transaction constituting settlement includes, but is not limited to, making lump-sum cash payment to participant in exchange for their rights to receive specified benefits and purchasing nonparticipating annuity contract. Excludes decreases to benefit obligation for remeasurement due to settlement.

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Amount, before benefit payment, of receipt from prescription drug subsidy which increases benefit obligation of defined benefit postretirement plan.

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Amount of increase in benefit obligation of defined benefit plan from business combination.

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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 (increase) decrease in benefit obligation of defined benefit plan from event reducing expected years of future service of present employees or eliminating accrual of benefits for some or all future services of present employees.

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Amount of cost recognized for passage of time related to defined benefit plan.

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Amount of increase (decrease) in benefit obligation of defined benefit plan from change in terms of existing plan or initiation of new plan.

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Amount of cost for actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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Amount of increase in benefit obligation for benefits provided to employees payable from defined benefit plan or payable upon retirement.

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