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Note 13 - Pension Plans and Other Postretirement Benefits - Other Changes in Plan Assets and Pension Benefit Obligations (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2021
Pension Plan [Member] |  |  |
Current year actuarial loss (gain) | $ 14.5 | $ (16.9) | $ (8.1)
Amortization of actuarial loss | (6.6) | (11.9) | (14.9)
Acceleration of loss recognized due to settlement | (9.7) | (33.6) | (18.2)
Current year service credit | 0.0 | 0.0 | (17.9)
Amortization of prior service credit | 4.5 | 4.5 | 3.1
Subtotal | 2.7 | (57.9) | (56.0)
Regulatory adjustment | (2.8) | 56.3 | 57.3
Total recognized in OCI | (0.1) | (1.6) | 1.3
Pension Plan [Member] | Spire Missouri [Member] |  |  |
Current year actuarial loss (gain) | 5.7 | 0.9 | (0.9)
Amortization of actuarial loss | (6.1) | (9.6) | (11.0)
Acceleration of loss recognized due to settlement | (0.6) | (27.3) | (11.6)
Current year service credit | 0.0 | 0.0 | (17.9)
Amortization of prior service credit | 1.9 | 1.9 | 0.6
Subtotal | 0.9 | (34.1) | (40.8)
Regulatory adjustment | (1.0) | 32.5 | 42.1
Total recognized in OCI | (0.1) | (1.6) | 1.3
Pension Plan [Member] | Spire Alabama Inc [Member] |  |  |
Current year actuarial loss (gain) | 9.0 | (16.0) | (1.5)
Amortization of actuarial loss | (0.9) | (2.4) | (3.9)
Acceleration of loss recognized due to settlement | (9.1) | (6.3) | (6.6)
Current year service credit | 0.0 | 0.0 | 0.0
Amortization of prior service credit | 2.4 | 2.4 | 2.3
Subtotal | 1.4 | (22.3) | (9.7)
Regulatory adjustment | (1.4) | 22.3 | 9.7
Total recognized in OCI | 0.0 | 0.0 | 0.0
Other Postretirement Benefits Plan [Member] |  |  |
Current year actuarial loss (gain) | (24.4) | 15.8 | (41.0)
Amortization of actuarial loss | 4.0 | 2.2 | 1.6
Amortization of prior service credit | (0.3) | (1.0) | (1.0)
Subtotal | (20.7) | 10.7 | (40.4)
Regulatory adjustment | 20.7 | (10.7) | 40.4
Total recognized in OCI | 0.0 | 0.0 | 0.0
Current year prior service cost | 0.0 | (6.3) | 0.0
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member] |  |  |
Current year actuarial loss (gain) | (21.5) | 9.8 | (29.0)
Amortization of actuarial loss | 3.2 | 1.9 | 1.5
Amortization of prior service credit | (0.6) | (0.7) | (0.7)
Subtotal | (18.9) | 9.9 | (28.2)
Regulatory adjustment | 18.9 | (9.9) | 28.2
Total recognized in OCI | 0.0 | 0.0 | 0.0
Current year prior service cost |  | (1.1) | 0.0
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member] |  |  |
Current year actuarial loss (gain) | (1.1) | 5.9 | (9.9)
Amortization of actuarial loss | 0.5 | 0.0 | 0.0
Amortization of prior service credit | 0.3 | (0.3) | (0.3)
Subtotal | (0.3) | 0.4 | (10.2)
Regulatory adjustment | 0.3 | (0.4) | 10.2
Total recognized in OCI | 0.0 | 0.0 | 0.0
Current year prior service cost | $ 0.0 | $ (5.2) | $ 0.0

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- Definition The difference between the period's net benefit cost recognized in other comprehensive income for defined benefit plans calculated in accordance with GAAP and the amount of recovery allowed in rates by the Missouri Public Service Commission. These differences are deferred as regulatory assets or regulatory liabilities. + References No definition available. + Details
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- Definition The aggregate net gain (loss) recognized in other comprehensive income, net of any regulatory adjustments, including amounts arising during the period and reclassification adjustments of other comprehensive income as a result of being recognized as components of net periodic benefit cost for the period. + References No definition available. + Details
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- Definition Amount, before tax, of 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 -Section 45 -Paragraph 10A -Subparagraph (j) -SubTopic 10 -Topic 220 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482790/220-10-45-10A Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 220 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 11 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482790/220-10-45-11 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (i) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Amount, before tax, of increase (decrease) to other comprehensive income from settlement and curtailment gain (loss) of defined benefit plan. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 45 -Paragraph 15 -SubTopic 10 -Topic 220 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482790/220-10-45-15 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 220 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 11 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482790/220-10-45-11 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (i) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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