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Note 13 - Pension Plans and Other Postretirement Benefits - Fair Value Measurements of Pension Plan Assets (Details) - USD ($) $ in Millions | Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2021
--- | --- | --- | ---
Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets |  | $ 235.9 | $ 364.0
Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets |  | 57.3 | 82.8
Pension Plan [Member] |  |  |
Defined Benefit Plan Assets | $ 334.7 | 332.7 | 498.9
Pension Plan [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 242.0 | 235.9 |
Pension Plan [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | 53.6 | 57.3 |
Pension Plan [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Defined Benefit Plan Assets | 78.8 | 89.9 |
Pension Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 56.4 | 59.7 |
Pension Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | 12.9 | 18.0 |
Pension Plan [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Defined Benefit Plan Assets | 255.9 | 242.8 |
Pension Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 185.6 | 176.2 |
Pension Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | 40.7 | 39.3 |
Pension Plan [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Defined Benefit Plan Assets | 334.7 | 332.7 |
Pension Plan [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 242.0 | 235.9 |
Pension Plan [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | 53.6 | 57.3 |
Pension Plan [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Defined Benefit Plan Assets | 14.8 | 11.5 |
Pension Plan [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 6.8 | 5.7 |
Pension Plan [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | 4.6 | 3.5 |
Pension Plan [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Defined Benefit Plan Assets | 0.0 | 0.0 |
Pension Plan [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 0.0 | 0.0 |
Pension Plan [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | 0.0 | 0.0 |
Pension Plan [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Defined Benefit Plan Assets | 14.8 | 11.5 |
Pension Plan [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 6.8 | 5.7 |
Pension Plan [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | 4.6 | 3.5 |
Pension Plan [Member] | Equity Funds [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Defined Benefit Plan Assets | 0.0 | 0.0 |
Pension Plan [Member] | Equity Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 0.0 | 0.0 |
Pension Plan [Member] | Equity Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | 0.0 | 0.0 |
Pension Plan [Member] | Equity Funds [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Defined Benefit Plan Assets | 139.0 | 122.2 |
Pension Plan [Member] | Equity Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 99.4 | 85.7 |
Pension Plan [Member] | Equity Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | 22.9 | 21.6 |
Pension Plan [Member] | Equity Funds [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Defined Benefit Plan Assets | 139.0 | 122.2 |
Pension Plan [Member] | Equity Funds [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 99.4 | 85.7 |
Pension Plan [Member] | Equity Funds [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | 22.9 | 21.6 |
Pension Plan [Member] | Real Asset Funds [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Defined Benefit Plan Assets | 0.0 | 0.0 |
Pension Plan [Member] | Real Asset Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 0.0 | 0.0 |
Pension Plan [Member] | Real Asset Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | 0.0 | 0.0 |
Pension Plan [Member] | Real Asset Funds [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Defined Benefit Plan Assets | 55.2 | 61.0 |
Pension Plan [Member] | Real Asset Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 40.2 | 47.2 |
Pension Plan [Member] | Real Asset Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | 8.7 | 8.1 |
Pension Plan [Member] | Real Asset Funds [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Defined Benefit Plan Assets | 55.2 | 61.0 |
Pension Plan [Member] | Real Asset Funds [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 40.2 | 47.2 |
Pension Plan [Member] | Real Asset Funds [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | 8.7 | 8.1 |
Pension Plan [Member] | U S Bond Funds [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Defined Benefit Plan Assets | 27.8 | 42.3 |
Pension Plan [Member] | U S Bond Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 20.9 | 29.4 |
Pension Plan [Member] | U S Bond Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | 4.0 | 7.7 |
Pension Plan [Member] | U S Bond Funds [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Defined Benefit Plan Assets | 0.0 | 0.0 |
Pension Plan [Member] | U S Bond Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 0.0 | 0.0 |
Pension Plan [Member] | U S Bond Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | 0.0 | 0.0 |
Pension Plan [Member] | U S Bond Funds [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Defined Benefit Plan Assets | 27.8 | 42.3 |
Pension Plan [Member] | U S Bond Funds [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 20.9 | 29.4 |
Pension Plan [Member] | U S Bond Funds [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | 4.0 | 7.7 |
Pension Plan [Member] | U S Government Index Funds [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Defined Benefit Plan Assets | 36.2 | 36.1 |
Pension Plan [Member] | U S Government Index Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 28.7 | 24.6 |
Pension Plan [Member] | U S Government Index Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | 4.3 | 6.8 |
Pension Plan [Member] | U S Government Index Funds [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Defined Benefit Plan Assets | 0.0 | 0.0 |
Pension Plan [Member] | U S Government Index Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 0.0 | 0.0 |
Pension Plan [Member] | U S Government Index Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | 0.0 | 0.0 |
Pension Plan [Member] | U S Government Index Funds [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Defined Benefit Plan Assets | 36.2 | 36.1 |
Pension Plan [Member] | U S Government Index Funds [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 28.7 | 24.6 |
Pension Plan [Member] | U S Government Index Funds [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | 4.3 | 6.8 |
Pension Plan [Member] | Global Funds Including U S [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Defined Benefit Plan Assets | 0.0 | 0.0 |
Pension Plan [Member] | Global Funds Including U S [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 0.0 | 0.0 |
Pension Plan [Member] | Global Funds Including U S [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | 0.0 | 0.0 |
Pension Plan [Member] | Global Funds Including U S [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Defined Benefit Plan Assets | 61.7 | 59.6 |
Pension Plan [Member] | Global Funds Including U S [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 46.0 | 43.3 |
Pension Plan [Member] | Global Funds Including U S [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | 9.1 | 9.6 |
Pension Plan [Member] | Global Funds Including U S [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Defined Benefit Plan Assets | 61.7 | 59.6 |
Pension Plan [Member] | Global Funds Including U S [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 46.0 | 43.3 |
Pension Plan [Member] | Global Funds Including U S [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | 9.1 | 9.6 |
Other Postretirement Benefits Plan [Member] |  |  |
Defined Benefit Plan Assets | 286.3 | 264.7 | 326.9
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 188.8 | 176.7 | 221.7
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | 92.0 | 83.3 | $ 99.4
Other Postretirement Benefits Plan [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Defined Benefit Plan Assets | 193.3 | 181.4 |
Other Postretirement Benefits Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 188.8 | 176.7 |
Other Postretirement Benefits Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | 0.0 | 0.0 |
Other Postretirement Benefits Plan [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Defined Benefit Plan Assets | 93.0 | 83.3 |
Other Postretirement Benefits Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 0.0 | 0.0 |
Other Postretirement Benefits Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | 92.0 | 83.3 |
Other Postretirement Benefits Plan [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Defined Benefit Plan Assets | 286.3 | 264.7 |
Other Postretirement Benefits Plan [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 188.8 | 176.7 |
Other Postretirement Benefits Plan [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | 92.0 | 83.3 |
Other Postretirement Benefits Plan [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Defined Benefit Plan Assets | 5.1 | 3.5 |
Other Postretirement Benefits Plan [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 4.3 | 2.7 |
Other Postretirement Benefits Plan [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Defined Benefit Plan Assets | 0.0 | 0.0 |
Other Postretirement Benefits Plan [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 0.0 | 0.0 |
Other Postretirement Benefits Plan [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Defined Benefit Plan Assets | 5.1 | 3.5 |
Other Postretirement Benefits Plan [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 4.3 | 2.7 |
Other Postretirement Benefits Plan [Member] | U S Stock Bond Mutual Fund [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Defined Benefit Plan Assets | 188.2 | 177.1 |
Other Postretirement Benefits Plan [Member] | U S Stock Bond Mutual Fund [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 184.5 | 174.0 |
Other Postretirement Benefits Plan [Member] | U S Stock Bond Mutual Fund [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | 0.0 | 0.0 |
Other Postretirement Benefits Plan [Member] | U S Stock Bond Mutual Fund [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Defined Benefit Plan Assets | 76.5 | 69.4 |
Other Postretirement Benefits Plan [Member] | U S Stock Bond Mutual Fund [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 0.0 | 0.0 |
Other Postretirement Benefits Plan [Member] | U S Stock Bond Mutual Fund [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | 76.5 | 69.4 |
Other Postretirement Benefits Plan [Member] | U S Stock Bond Mutual Fund [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Defined Benefit Plan Assets | 264.7 | 246.5 |
Other Postretirement Benefits Plan [Member] | U S Stock Bond Mutual Fund [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Missouri [Member] |  |  |
Defined Benefit Plan Assets | 184.5 | 174.0 |
Other Postretirement Benefits Plan [Member] | U S Stock Bond Mutual Fund [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | 76.5 | 69.4 |
Other Postretirement Benefits Plan [Member] | Debt Security, Corporate, Non-US [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Defined Benefit Plan Assets | 0.0 | 0.8 |
Other Postretirement Benefits Plan [Member] | Debt Security, Corporate, Non-US [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | 0.0 | 0.0 |
Other Postretirement Benefits Plan [Member] | Debt Security, Corporate, Non-US [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Defined Benefit Plan Assets | 16.5 | 13.9 |
Other Postretirement Benefits Plan [Member] | Debt Security, Corporate, Non-US [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | 15.5 | 13.9 |
Other Postretirement Benefits Plan [Member] | Debt Security, Corporate, Non-US [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Defined Benefit Plan Assets | 16.5 | 14.7 |
Other Postretirement Benefits Plan [Member] | Debt Security, Corporate, Non-US [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Alabama Inc [Member] |  |  |
Defined Benefit Plan Assets | $ 15.5 | $ 13.9 |

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