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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

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- Definition Amounts of adjustments for defined benefit plan included in regulatory assets. + References No definition available. + Details
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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 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 45 -Paragraph 10A -Subparagraph (j) -SubTopic 10 -Topic 220 -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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