PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Benefit Obligation Rollforward (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Pension Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	$ 794.7	$ 652.3
Service cost	20.5	15.3	$ 17.3
Interest cost	27.9	28.0	29.5
Actuarial (gain) loss	(0.9)	85.8
Plan amendments	(20.7)	5.1
Spire EnergySouth acquisition	0.0	60.4
Settlement loss	14.6	1.1
Special termination benefits	0.9	1.6
Settlement benefits paid	(62.2)	(16.6)	(71.1)
Regular benefits paid	(26.0)	(38.3)
Benefit obligation, end of year	748.8	794.7	652.3
Accumulated benefit obligation, end of year	701.4	724.5
Postretirement Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	259.2	239.2
Service cost	11.0	10.9	12.8
Interest cost	8.6	10.2	11.2
Actuarial (gain) loss	(22.1)	7.1
Plan amendments	(1.4)	(1.8)
Spire EnergySouth acquisition	0.0	5.9
Special termination benefits	0.0	2.6
Curtailments	0.4	0.0
Retiree drug subsidy program	0.3	0.2
Regular benefits paid	(17.5)	(15.1)
Benefit obligation, end of year	238.5	259.2	239.2
Spire Missouri \| Pension Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	560.0	497.6
Service cost	12.7	10.0	11.5
Interest cost	19.5	21.7	23.3
Actuarial (gain) loss	(0.5)	59.2
Plan amendments	0.0	5.1
Spire EnergySouth acquisition	0.0	0.0
Settlement loss	12.2	0.0
Special termination benefits	0.0	1.6
Settlement benefits paid	(43.5)	0.0	(58.2)
Regular benefits paid	(20.8)	(35.2)
Benefit obligation, end of year	539.6	560.0	497.6
Accumulated benefit obligation, end of year	500.4	517.7
Spire Missouri \| Postretirement Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	207.9	191.9
Service cost	10.4	10.6	12.3
Interest cost	6.8	8.1	8.6
Actuarial (gain) loss	(20.9)	6.7
Plan amendments	0.0	0.0
Spire EnergySouth acquisition	0.0	0.0
Special termination benefits	0.0	2.6
Curtailments	0.0	0.0
Retiree drug subsidy program	0.3	0.0
Regular benefits paid	(12.0)	(12.0)
Benefit obligation, end of year	192.5	207.9	191.9
Spire Alabama \| Pension Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	174.3	154.7
Service cost	6.2	5.3	5.8
Interest cost	6.1	6.3	6.2
Actuarial (gain) loss	1.6	26.6
Plan amendments	(20.7)	0.0
Spire EnergySouth acquisition	0.0	0.0
Settlement loss	2.4	1.1
Special termination benefits	0.0	0.0
Settlement benefits paid	(18.7)	(16.6)	(12.9)
Regular benefits paid	(3.0)	(3.1)
Benefit obligation, end of year	148.2	174.3	154.7
Accumulated benefit obligation, end of year	142.8	149.8
Spire Alabama \| Postretirement Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	45.4	47.3
Service cost	0.3	0.3	0.5
Interest cost	1.6	2.1	2.6
Actuarial (gain) loss	0.0	0.4
Plan amendments	(1.4)	(1.8)
Spire EnergySouth acquisition	0.0	0.0
Special termination benefits	0.0	0.0
Curtailments	0.0	0.0
Retiree drug subsidy program	0.0	0.2
Regular benefits paid	(5.3)	(3.1)
Benefit obligation, end of year	$ 40.6	$ 45.4	$ 47.3

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

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 60
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=6414203&loc=d3e39716-114964

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-Publisher FASB
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-Section 50
-Paragraph 1
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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 30
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-URI http://asc.fasb.org/extlink&oid=7577095&loc=d3e8001-114927

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-Publisher FASB
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- Definition

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

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- Definition

Amount of increase in benefit obligation 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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No definition available.

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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
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-Publisher FASB
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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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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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