Pension Plans and Other Postretirement Benefits - Assumptions Used to Calculate Net Periodic Cost and Benefit Obligations (Details)	12 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Pension Plans
Assumptions used to calculate benefit obligations [Abstract]
Discount rates	3.00%	3.00%
Pension Plans \| Spire Missouri East
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate	3.20%	4.30%	3.75%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate	2.85%	3.20%
Pension Plans \| Spire Missouri West
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate	3.15%	4.35%	3.70%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate	2.75%	3.15%
Pension Plans \| Spire Missouri
Assumptions used to calculate net periodic cost [Abstract]
Weighted average rate of future compensation increase	3.00%	3.00%	3.00%
Expected long-term rate of return on plan assets	7.25%	7.50%	7.75%
Assumptions used to calculate benefit obligations [Abstract]
Discount rates	2.85%	3.20%
Expected long-term rate of return on plan assets	7.25%	7.50%	7.75%
Pension Plans \| Spire Alabama
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate		4.35%
Weighted average rate of future compensation increase	3.00%	3.00%	3.00%
Expected long-term rate of return on plan assets	7.25%	7.25%	7.25%
Assumptions used to calculate benefit obligations [Abstract]
Discount rates	2.95%	3.20%
Expected long-term rate of return on plan assets	7.25%	7.25%	7.25%
Pension Plans \| Spire Alabama \| Maximum
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate	3.25%		3.70%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate	2.95%	3.25%
Pension Plans \| Spire Alabama \| Minimum [Member]
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate	3.20%		3.65%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate	2.80%	3.20%
Other Postretirement Plans \| Spire Missouri East
Assumptions used to calculate benefit obligations [Abstract]
Discount rates	3.00%	3.00%
Other Postretirement Plans \| Spire Missouri
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate	3.15%	4.30%	3.60%
Weighted average rate of future compensation increase	3.00%	3.00%	3.00%
Expected long-term rate of return on plan assets	6.25%	6.25%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate	2.75%	3.15%
Expected long-term rate of return on plan assets	6.25%	6.25%
Other Postretirement Plans \| Spire Missouri \| Maximum
Assumptions used to calculate net periodic cost [Abstract]
Expected long-term rate of return on plan assets			7.75%
Assumptions used to calculate benefit obligations [Abstract]
Expected long-term rate of return on plan assets			7.75%
Other Postretirement Plans \| Spire Missouri \| Minimum [Member]
Assumptions used to calculate net periodic cost [Abstract]
Expected long-term rate of return on plan assets			5.75%
Assumptions used to calculate benefit obligations [Abstract]
Expected long-term rate of return on plan assets			5.75%
Other Postretirement Plans \| Spire Alabama
Assumptions used to calculate net periodic cost [Abstract]
Weighted average discount rate	3.15%	4.30%	3.80%
Assumptions used to calculate benefit obligations [Abstract]
Weighted average discount rate	2.75%	3.15%
Other Postretirement Plans \| Spire Alabama \| Maximum
Assumptions used to calculate net periodic cost [Abstract]
Expected long-term rate of return on plan assets	6.25%	6.25%	6.00%
Assumptions used to calculate benefit obligations [Abstract]
Expected long-term rate of return on plan assets	6.25%	6.25%	6.00%
Other Postretirement Plans \| Spire Alabama \| Minimum [Member]
Assumptions used to calculate net periodic cost [Abstract]
Expected long-term rate of return on plan assets	5.00%	5.00%	3.75%
Assumptions used to calculate benefit obligations [Abstract]
Expected long-term rate of return on plan assets	5.00%	5.00%	3.75%

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- Definition

Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of defined benefit plan.

+ References

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- Definition

Weighted average rate increase of compensation, used to determine benefit obligation of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan.

+ References

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- Definition

Weighted average rate for present value of future retirement benefits cash flows, used to determine net periodic benefit cost of defined benefit plan.

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- Definition

Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (k)(3)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2919-114920

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- Definition

Weighted average rate of compensation increase used to determine net periodic benefit cost of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan.

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