PENSION PLANS AND OTHER POSTRETIREMENT BENEFITS - Expected Future Benefit Payments (Details) $ in Millions	Sep. 30, 2018 USD ($)
Pension Plans
Expected benefit payments [Abstract]
2019	$ 63.0
2020	63.6
2021	60.0
2022	58.4
2023	57.1
2024 - 2028	266.4
Postretirement Plans
Expected benefit payments [Abstract]
2019	14.2
2020	15.3
2021	16.3
2022	16.9
2023	17.9
2024 - 2028	93.9
Spire Missouri \| Pension Plans
Expected benefit payments [Abstract]
2019	50.3
2020	50.2
2021	46.0
2022	43.8
2023	41.9
2024 - 2028	184.8
Spire Missouri \| Postretirement Plans
Expected benefit payments [Abstract]
2019	10.8
2020	11.8
2021	12.8
2022	13.4
2023	14.3
2024 - 2028	76.4
Spire Alabama \| Pension Plans
Expected benefit payments [Abstract]
2019	10.0
2020	10.7
2021	11.2
2022	11.8
2023	12.3
2024 - 2028	65.5
Spire Alabama \| Postretirement Plans
Expected benefit payments [Abstract]
2019	3.0
2020	3.1
2021	3.1
2022	3.1
2023	3.2
2024 - 2028	$ 15.1

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Amount of benefits for defined benefit plan expected to be paid in five fiscal years after fifth fiscal year following latest fiscal year.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (f)
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Amount of benefits for defined benefit plan expected to be paid in next fiscal year following latest fiscal year.

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Amount of benefits for defined benefit plan expected to be paid in third fiscal year following latest fiscal year.

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Amount of benefits for defined benefit plan expected to be paid in second fiscal year following latest fiscal year.

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