Pension Plans and Other Postretirement Benefits - Fair Value Measurements of Pension Plan Assets (Details) - USD ($) $ in Millions	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Pension Plan [Member]
Defined Benefit Plan Assets	$ 427.7	$ 374.8	$ 334.7
Pension Plan [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	284.8	259.8	242.0
Pension Plan [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	90.6	69.9	53.6
Pension Plan [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Assets	116.4	102.2
Pension Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	75.4	73.3
Pension Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	25.3	17.5
Pension Plan [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Assets	311.3	272.6
Pension Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	209.4	186.5
Pension Plan [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	65.3	52.4
Pension Plan [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Assets	427.7	374.8
Pension Plan [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	284.8	259.8
Pension Plan [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	90.6	69.9
Pension Plan [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Assets	27.4	12.7
Pension Plan [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	10.1	9.9
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	10.2	1.7
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	27.4	12.7
Pension Plan [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	10.1	9.9
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	10.2	1.7
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	181.5	152.4
Pension Plan [Member] | Equity Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	117.1	102.2
Pension Plan [Member] | Equity Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	41.3	30.6
Pension Plan [Member] | Equity Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Assets	181.5	152.4
Pension Plan [Member] | Equity Funds [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	117.1	102.2
Pension Plan [Member] | Equity Funds [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	41.3	30.6
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	61.4	57.7
Pension Plan [Member] | Real Asset Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	43.1	40.2
Pension Plan [Member] | Real Asset Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	11.7	10.6
Pension Plan [Member] | Real Asset Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Assets	61.4	57.7
Pension Plan [Member] | Real Asset Funds [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	43.1	40.2
Pension Plan [Member] | Real Asset Funds [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	11.7	10.6
Pension Plan [Member] | U S Bond Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Assets	50.7	55.6
Pension Plan [Member] | U S Bond Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	35.1	38.2
Pension Plan [Member] | U S Bond Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	9.9	10.6
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	50.7	55.6
Pension Plan [Member] | U S Bond Funds [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	35.1	38.2
Pension Plan [Member] | U S Bond Funds [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	9.9	10.6
Pension Plan [Member] | U S Government Index Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Assets	38.3	33.9
Pension Plan [Member] | U S Government Index Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	30.2	25.2
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	5.2
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	38.3	33.9
Pension Plan [Member] | U S Government Index Funds [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	30.2	25.2
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	5.2
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	68.4	62.5
Pension Plan [Member] | Global Funds Including U S [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	49.2	44.1
Pension Plan [Member] | Global Funds Including U S [Member] | Fair Value, Inputs, Level 2 [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	12.3	11.2
Pension Plan [Member] | Global Funds Including U S [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Assets	68.4	62.5
Pension Plan [Member] | Global Funds Including U S [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	49.2	44.1
Pension Plan [Member] | Global Funds Including U S [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	12.3	11.2
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Assets	372.9	339.4	286.3
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	251.4	224.9	188.8
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	113.5	107.5	$ 92.0
Other Postretirement Benefits Plan [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Assets	309.0	230.7
Other Postretirement Benefits Plan [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	251.4	224.9
Other Postretirement Benefits Plan [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Assets	63.9	108.7
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 3 [Member]
Defined Benefit Plan Assets	372.9	339.4
Other Postretirement Benefits Plan [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Missouri [Member]
Defined Benefit Plan Assets	251.4	224.9
Other Postretirement Benefits Plan [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Assets	13.1	1.6
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	12.1	0.7
Other Postretirement Benefits Plan [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 1 [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	49.6	0.0
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 2 [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	63.9	107.5
Other Postretirement Benefits Plan [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Assets	13.1	1.6
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	12.1	0.7
Other Postretirement Benefits Plan [Member] | Defined Benefit Plan, Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 3 [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Assets	113.5	107.5
Other Postretirement Benefits Plan [Member] | U S Stock Bond Mutual Fund [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Assets	294.5	229.1
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	239.3	224.2
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	49.6	0.0
Other Postretirement Benefits Plan [Member] | U S Stock Bond Mutual Fund [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Assets	44.4	89.0
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	44.4	89.0
Other Postretirement Benefits Plan [Member] | U S Stock Bond Mutual Fund [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Assets	338.9	318.1
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	239.3	224.2
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	94.0	89.0
Other Postretirement Benefits Plan [Member] | Debt Security, Corporate, Non-US [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Assets	1.4	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	19.5	19.7
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	19.5	18.5
Other Postretirement Benefits Plan [Member] | Debt Security, Corporate, Non-US [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Assets	20.9	19.7
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	$ 19.5	$ 18.5