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