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Note 13 - Pension Plans and Other Postretirement Benefits - Reconciliation of Fair Value of Plan Assets (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2023 | Sep. 30, 2022
Spire Missouri [Member] |  |
Fair value of plan assets, beginning of year | $ 235.9 | $ 364.0
Actual return on plan assets |  | (66.7)
Employer contributions |  | 37.8
Settlement benefits paid |  | (87.0)
Regular benefits paid |  | (12.2)
Fair value of plan assets, end of year |  | 235.9
Funded status of plans, end of year |  | (84.7)
Spire Alabama Inc [Member] |  |
Fair value of plan assets, beginning of year | 57.3 | 82.8
Actual return on plan assets |  | (15.6)
Employer contributions |  | 14.4
Settlement benefits paid |  | (22.3)
Regular benefits paid |  | (2.0)
Fair value of plan assets, end of year |  | 57.3
Funded status of plans, end of year |  | (40.5)
Pension Plan [Member] |  |
Fair value of plan assets, beginning of year | 332.7 | 498.9
Actual return on plan assets | 9.1 | (92.5)
Employer contributions | 58.9 | 52.4
Settlement benefits paid | (39.1) | (109.3)
Regular benefits paid | (26.9) | (16.8)
Fair value of plan assets, end of year | 334.7 | 332.7
Funded status of plans, end of year | (104.0) | (131.7)
Pension Plan [Member] | Spire Missouri [Member] |  |
Fair value of plan assets, beginning of year | 235.9 |
Actual return on plan assets | 7.5 |
Employer contributions | 30.3 |
Settlement benefits paid | (9.3) |
Regular benefits paid | (22.4) |
Fair value of plan assets, end of year | 242.0 | 235.9
Funded status of plans, end of year | (71.2) |
Pension Plan [Member] | Spire Alabama Inc [Member] |  |
Fair value of plan assets, beginning of year | 57.3 |
Actual return on plan assets | 0.0 |
Employer contributions | 27.9 |
Settlement benefits paid | (29.7) |
Regular benefits paid | (1.9) |
Fair value of plan assets, end of year | 53.6 | 57.3
Funded status of plans, end of year | (27.0) |
Other Postretirement Benefits Plan [Member] |  |
Fair value of plan assets, beginning of year | 264.7 | 326.9
Actual return on plan assets | 29.9 | (51.6)
Employer contributions | 0.1 | 0.4
Regular benefits paid | (8.4) | (11.0)
Fair value of plan assets, end of year | 286.3 | 264.7
Funded status of plans, end of year | 143.7 | 116.6
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member] |  |
Fair value of plan assets, beginning of year | 176.7 | 221.7
Actual return on plan assets | 18.8 | (36.2)
Employer contributions | 0.1 | 0.4
Regular benefits paid | (6.8) | (9.2)
Fair value of plan assets, end of year | 188.8 | 176.7
Funded status of plans, end of year | 83.5 | 61.9
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member] |  |
Fair value of plan assets, beginning of year | 83.3 | 99.4
Actual return on plan assets | 10.2 | (14.3)
Employer contributions | 0.0 | 0.0
Regular benefits paid | (1.5) | (1.8)
Fair value of plan assets, end of year | 92.0 | 83.3
Funded status of plans, end of year | $ 56.9 | $ 53.4

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- Definition Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 17 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480482/715-20-55-17 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (b)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Amount of contribution received by defined benefit plan from employer which increases plan assets. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 17 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480482/715-20-55-17 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 18 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480482/715-20-55-18 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-6 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (b)(3) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 Reference 6: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 Reference 7: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 17 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480482/715-20-55-17 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(02) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 Reference 6: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 Reference 7: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Amount of funded (unfunded) status of defined benefit plan, measured as difference between fair value of plan assets and benefit obligation. Includes, but is not limited to, overfunded (underfunded) status. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 17 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480482/715-20-55-17 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 45 -Paragraph 2 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480535/715-20-45-2 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Amount of payment to participant under defined benefit plan which decreases plan assets. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 715 -SubTopic 60 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480266/715-60-50-4 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (b)(5) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Amount of payment, which decreases plan assets of defined benefit plan, for irrevocable action relieving primary responsibility for benefit obligation and eliminating risk for obligation and assets used to effect settlement. Transaction constituting settlement includes, but is not limited to, making lump-sum cash payment to participant in exchange for their rights to receive specified benefits and purchasing nonparticipating annuity contract. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (b)(8) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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