Pension Plans and Other Postretirement Benefits - Amounts Recognized in AOCI Not Yet Recognized As Components of Net Periodic Pension Costs (Details) - USD ($) $ in Millions	Sep. 30, 2025	Sep. 30, 2024
Pension Plan [Member]
Net actuarial loss	$ 100.9	$ 119.7
Prior service credit	(22.0)	(26.4)
Subtotal	78.9	93.3
Adjustments for amounts included in regulatory assets	(77.8)	(90.8)
Total	1.1	2.5
Pension Plan [Member] \| Spire Missouri [Member]
Net actuarial loss	58.5	72.9
Prior service credit	(12.1)	(14.0)
Subtotal	46.4	58.9
Adjustments for amounts included in regulatory assets	(45.3)	(56.4)
Total	1.1	2.5
Pension Plan [Member] \| Spire Alabama Inc [Member]
Net actuarial loss	44.5	47.7
Prior service credit	(9.1)	(11.5)
Subtotal	35.4	36.2
Adjustments for amounts included in regulatory assets	(35.4)	(36.2)
Total	0.0	0.0
Other Postretirement Benefits Plan [Member]
Net actuarial loss	(180.6)	(150.0)
Prior service credit	2.0	2.0
Subtotal	(178.6)	(148.0)
Adjustments for amounts included in regulatory assets	178.6	148.0
Total	0.0	0.0
Other Postretirement Benefits Plan [Member] \| Spire Missouri [Member]
Net actuarial loss	(154.2)	(127.5)
Prior service credit	2.0	2.1
Subtotal	(152.2)	(125.4)
Adjustments for amounts included in regulatory assets	152.2	125.4
Total	0.0	0.0
Other Postretirement Benefits Plan [Member] \| Spire Alabama Inc [Member]
Net actuarial loss	(19.3)	(15.4)
Prior service credit	0.0	(0.1)
Subtotal	(19.3)	(15.5)
Adjustments for amounts included in regulatory assets	19.3	15.5
Total	$ 0.0	$ 0.0

Amounts of adjustments for defined benefit plan included in regulatory assets.

No definition available.

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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.

+ References

No definition available.

+ Details

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- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 14A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482790/220-10-45-14A

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-Name Accounting Standards Codification
-Section 45
-Paragraph 10A
-Subparagraph (j)
-SubTopic 10
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482790/220-10-45-10A

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 10A
-Subparagraph (k)
-SubTopic 10
-Topic 220
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482790/220-10-45-10A

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 10A
-Subparagraph (i)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482790/220-10-45-10A

Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 715
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (j)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480506/715-20-50-1

+ Details

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- Definition

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.

+ References

+ Details

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- Definition

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).

+ References

+ Details

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