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Note 13 - 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, 2023 | Sep. 30, 2022
--- | --- | ---
Spire Missouri [Member] |  |
Net actuarial loss |  | $ 93.6
Prior service credit |  | (18.1)
Subtotal |  | 75.5
Adjustments for amounts included in regulatory assets |  | (72.3)
Total |  | 3.2
Spire Alabama Inc [Member] |  |
Net actuarial loss |  | 41.2
Prior service credit |  | (16.3)
Subtotal |  | 24.9
Adjustments for amounts included in regulatory assets |  | (24.9)
Total |  | 0.0
Pension Plan [Member] |  |
Net actuarial loss | $ 130.1 | 132.0
Prior service credit | (31.2) | (35.7)
Subtotal | 98.9 | 96.3
Adjustments for amounts included in regulatory assets | (95.9) | (93.1)
Total | 3.0 | 3.2
Pension Plan [Member] | Spire Missouri [Member] |  |
Net actuarial loss | 92.5 |
Prior service credit | (16.2) |
Subtotal | 76.3 |
Adjustments for amounts included in regulatory assets | (73.3) |
Total | 3.0 |
Pension Plan [Member] | Spire Alabama Inc [Member] |  |
Net actuarial loss | 40.2 |
Prior service credit | (13.9) |
Subtotal | 26.3 |
Adjustments for amounts included in regulatory assets | (26.3) |
Total | 0.0 |
Other Postretirement Benefits Plan [Member] |  |
Net actuarial loss | (103.3) | (83.3)
Prior service credit | 7.0 | 7.1
Subtotal | (96.3) | (76.2)
Adjustments for amounts included in regulatory assets | 96.3 | 76.2
Total | 0.0 | 0.0
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member] |  |
Net actuarial loss | (92.3) | (74.2)
Prior service credit | 7.4 | 7.9
Subtotal | (84.9) | (66.3)
Adjustments for amounts included in regulatory assets | 84.9 | 66.3
Total | 0.0 | 0.0
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member] |  |
Net actuarial loss | (6.1) | (5.6)
Prior service credit | (0.4) | (0.8)
Subtotal | (6.5) | (6.4)
Adjustments for amounts included in regulatory assets | 6.5 | 6.4
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 (i-k) -SubTopic 10 -Topic 220 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482790/220-10-45-10A Reference 3: 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 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://www.xbrl.org/2003/role/disclosureRef -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 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 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://www.xbrl.org/2003/role/disclosureRef -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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