PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Amounts Recognized in Consolidated Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent liabilities	$ (180.2)	$ (237.4)
Pension Plans
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Current liabilities	(0.6)	(0.5)
Noncurrent liabilities	(164.8)	(216.7)
Total	(165.4)	(217.2)
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	132.1	163.0
Prior service (credit) cost	(14.4)	(13.4)
Subtotal	117.7	149.6
Adjustments for amounts included in regulatory assets	(115.5)	(147.1)
Total	2.2	2.5
Postretirement Plans
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Current liabilities	(0.5)	(0.4)
Noncurrent liabilities	(16.1)	(21.0)
Total	75.4	27.0
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	(44.4)	1.5
Prior service (credit) cost	(6.6)	(6.6)
Subtotal	(51.0)	(5.1)
Adjustments for amounts included in regulatory assets	51.0	5.1
Total	0.0	0.0
Spire Missouri
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent liabilities	(136.9)	(173.0)
Spire Missouri \| Pension Plans
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Current liabilities	(0.6)	(0.5)
Noncurrent liabilities	(123.6)	(153.2)
Total	(124.2)	(153.7)
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	102.8	126.2
Prior service (credit) cost	6.4	7.3
Subtotal	109.2	133.5
Adjustments for amounts included in regulatory assets	(107.0)	(131.0)
Total	2.2	2.5
Spire Missouri \| Postretirement Plans
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Current liabilities	(0.5)	(0.4)
Noncurrent liabilities	(14.3)	(19.8)
Total	30.7	(17.6)
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	(35.7)	12.3
Prior service (credit) cost	(4.0)	(3.7)
Subtotal	(39.7)	8.6
Adjustments for amounts included in regulatory assets	39.7	(8.6)
Total	0.0	0.0
Spire Alabama
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent liabilities	(35.0)	(50.2)
Spire Alabama \| Pension Plans
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Current liabilities	0.0	0.0
Noncurrent liabilities	(35.0)	(50.3)
Total	(35.0)	(50.3)
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	36.4	40.9
Prior service (credit) cost	(18.8)	(20.7)
Subtotal	17.6	20.2
Adjustments for amounts included in regulatory assets	(17.6)	(20.2)
Total	0.0	0.0
Spire Alabama \| Postretirement Plans
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Current liabilities	0.0	0.0
Noncurrent liabilities	0.0	0.0
Total	46.5	45.8
Pre-tax amounts recognized in accumulated other comprehensive income not yet recognized as components of net periodic pension cost consist of:
Net actuarial loss	(8.0)	(9.7)
Prior service (credit) cost	(2.6)	(2.9)
Subtotal	(10.6)	(12.6)
Adjustments for amounts included in regulatory assets	10.6	12.6
Total	0.0	0.0
Current Asset [Member] \| Postretirement Plans
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent assets	0.0	1.4
Current Asset [Member] \| Spire Missouri \| Postretirement Plans
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent assets	0.0	1.4
Current Asset [Member] \| Spire Alabama \| Postretirement Plans
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent assets	0.0	0.0
Noncurrent Asset [Member] \| Postretirement Plans
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent assets	92.0	47.0
Noncurrent Asset [Member] \| Spire Missouri \| Postretirement Plans
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent assets	45.5	1.2
Noncurrent Asset [Member] \| Spire Alabama \| Postretirement Plans
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]
Noncurrent assets	$ 46.5	$ 45.8

Adjustments for defined benefit plan amount included in regulatory assets.

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- Definition

The pretax total, net of regulatory adjustments, of net gain (loss), prior service cost (credit), and transition assets (obligations), included in accumulated other comprehensive income associated with a defined benefit pension or other postretirement plan(s) because they have yet to be recognized as components of net periodic benefit cost net of adjustments for amounts included in regulatory assets.

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Amount, before tax, of accumulated other comprehensive (income) loss for defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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Amount, before tax, of accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

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Amount, before tax, of accumulated other comprehensive (income) loss for cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation of defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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Amount of asset (liability), recognized in statement of financial position, for defined benefit pension and other postretirement plans.

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Amount of asset, recognized in statement of financial position, for overfunded defined benefit pension and other postretirement plans.

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Amount of liability, recognized in statement of financial position, for defined benefit pension and other postretirement plans, classified as current.

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Amount of liability, recognized in statement of financial position, for defined benefit pension and other postretirement plans, classified as noncurrent.

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