Note 13 - Pension Plans and Other Postretirement Benefits - Fair Value Measurements of Pension Plan Assets (Details) - USD ($) $ in Millions	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Spire Missouri [Member]
Defined Benefit Plan Assets		$ 242.0	$ 235.9
Spire Alabama Inc [Member]
Defined Benefit Plan Assets		53.6	57.3
Pension Plan [Member]
Defined Benefit Plan Assets	$ 374.8	334.7	332.7
Pension Plan [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	259.8	242.0
Pension Plan [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	69.9	53.6
Pension Plan [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Assets	102.2	78.8
Pension Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	73.3	56.4
Pension Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	17.5	12.9
Pension Plan [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Assets	272.6	255.9
Pension Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	186.5	185.6
Pension Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	52.4	40.7
Pension Plan [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Assets	374.8	334.7
Pension Plan [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	259.8	242.0
Pension Plan [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	69.9	53.6
Pension Plan [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Assets	12.7	14.8
Pension Plan [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	9.9	6.8
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	1.7	4.6
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	12.7	14.8
Pension Plan [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	9.9	6.8
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	1.7	4.6
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	152.4	139.0
Pension Plan [Member] | Equity Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	102.2	99.4
Pension Plan [Member] | Equity Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	30.6	22.9
Pension Plan [Member] | Equity Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Assets	152.4	139.0
Pension Plan [Member] | Equity Funds [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	102.2	99.4
Pension Plan [Member] | Equity Funds [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	30.6	22.9
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	57.7	55.2
Pension Plan [Member] | Real Asset Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	40.2	40.2
Pension Plan [Member] | Real Asset Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	10.6	8.7
Pension Plan [Member] | Real Asset Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Assets	57.7	55.2
Pension Plan [Member] | Real Asset Funds [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	40.2	40.2
Pension Plan [Member] | Real Asset Funds [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	10.6	8.7
Pension Plan [Member] | U S Bond Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Assets	55.6	27.8
Pension Plan [Member] | U S Bond Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	38.2	20.9
Pension Plan [Member] | U S Bond Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	10.6	4.0
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	55.6	27.8
Pension Plan [Member] | U S Bond Funds [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	38.2	20.9
Pension Plan [Member] | U S Bond Funds [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	10.6	4.0
Pension Plan [Member] | U S Government Index Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Assets	33.9	36.2
Pension Plan [Member] | U S Government Index Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	25.2	28.7
Pension Plan [Member] | U S Government Index Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	5.2	4.3
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	33.9	36.2
Pension Plan [Member] | U S Government Index Funds [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	25.2	28.7
Pension Plan [Member] | U S Government Index Funds [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	5.2	4.3
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	62.5	61.7
Pension Plan [Member] | Global Funds Including U S [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	44.1	46.0
Pension Plan [Member] | Global Funds Including U S [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	11.2	9.1
Pension Plan [Member] | Global Funds Including U S [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Assets	62.5	61.7
Pension Plan [Member] | Global Funds Including U S [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	44.1	46.0
Pension Plan [Member] | Global Funds Including U S [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	11.2	9.1
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Assets	339.4	286.3	264.7
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	224.9	188.8	176.7
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	107.5	92.0	$ 83.3
Other Postretirement Benefits Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	224.9	188.8
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] | 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	107.5	92.0
Other Postretirement Benefits Plan [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	224.9	188.8
Other Postretirement Benefits Plan [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	107.5	92.0
Other Postretirement Benefits Plan [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Assets	1.6	5.1
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	0.7	4.3
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	1.6	5.1
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	0.7	4.3
Other Postretirement Benefits Plan [Member] | U S Stock Bond Mutual Fund [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Assets	229.1	188.2
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	224.2	184.5
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	89.0	76.5
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	89.0	76.5
Other Postretirement Benefits Plan [Member] | U S Stock Bond Mutual Fund [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Assets	318.1	264.7
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	224.2	184.5
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	89.0	76.5
Other Postretirement Benefits Plan [Member] | Debt Security, Corporate, Non-US [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Assets	0.0	0.0
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	19.7	16.5
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	18.5	15.5
Other Postretirement Benefits Plan [Member] | Debt Security, Corporate, Non-US [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Assets	19.7	16.5
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	$ 18.5	$ 15.5