Regulatory Matters - Schedule of Regulatory Assets and Liabilities, Reflected in Balance Sheets (Details) - USD ($) $ in Millions	Mar. 31, 2022	Sep. 30, 2021	Mar. 31, 2021
Regulatory Asset And Liabilities [Line Items]
Regulatory assets	$ 164.0	$ 306.5	$ 60.4
Regulatory assets, non current	1,106.8	993.5	1,133.6
Total Regulatory Assets	1,270.8	1,300.0	1,194.0
Regulatory liabilities	3.2	34.6	56.3
Regulatory liabilities, non current	400.4	620.9	423.5
Total Regulatory Liabilities	403.6	655.5	479.8
Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets	112.7	276.3	37.5
Regulatory assets, non current	572.7	483.1	604.3
Total Regulatory Assets	685.4	759.4	641.8
Regulatory liabilities	0.0	17.1	37.6
Regulatory liabilities, non current	313.7	538.8	353.5
Total Regulatory Liabilities	313.7	555.9	391.1
Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets	41.0	18.8	11.8
Regulatory assets, non current	506.7	483.3	501.2
Total Regulatory Assets	547.7	502.1	513.0
Regulatory liabilities	0.0	13.4	14.2
Regulatory liabilities, non current	24.7	23.4	18.0
Total Regulatory Liabilities	24.7	36.8	32.2
Pension and Other Postretirement Benefits
Regulatory Asset And Liabilities [Line Items]
Regulatory assets	0.0	31.1	30.9
Regulatory assets, non current	314.8	313.8	418.3
Pension and Other Postretirement Benefits | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets	0.0	21.9	21.9
Regulatory assets, non current	227.5	226.0	324.6
Pension and Other Postretirement Benefits | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets		8.2	8.0
Regulatory assets, non current	82.0	82.9	88.5
Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory assets	146.6	243.5	0.1
Regulatory assets, non current	35.2	0.0	51.5
Unamortized Purchased Gas Adjustments | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets	112.5	242.8	0.0
Regulatory assets, non current	35.2	0.0	51.5
Unamortized Purchased Gas Adjustments | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets	31.0	0.0	0.0
Other
Regulatory Asset And Liabilities [Line Items]
Regulatory assets	17.4	31.9	29.4
Regulatory assets, non current	109.4	67.3	62.5
Other | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets	0.2	11.6	15.6
Regulatory assets, non current	92.7	50.4	45.1
Other | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets	10.0	10.6	3.8
Regulatory assets, non current	1.1	1.2	1.1
Future Income Taxes Due from Customers
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	138.0	132.9	127.4
Future Income Taxes Due from Customers | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	129.3	124.2	118.6
Future Income Taxes Due from Customers | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	2.2	2.2	2.2
Accrued Cost of Removal
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	456.3	431.9	430.7
Accrued Cost of Removal | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	34.9	34.9	21.3
Accrued Cost of Removal | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	421.4	397.0	409.4
Energy Efficiency
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	53.1	47.6	43.2
Energy Efficiency | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	53.1	47.6	43.2
Pension and Other Postretirement Benefits
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities	0.0	5.8	5.8
Regulatory liabilities, non current	179.4	159.3	170.2
Pension and Other Postretirement Benefits | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities	0.0	3.6	3.6
Regulatory liabilities, non current	148.3	131.4	151.9
Pension and Other Postretirement Benefits | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities		2.2	2.2
Regulatory liabilities, non current	21.1	19.8	14.3
Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities	0.0	11.0	28.9
Regulatory liabilities, non current	17.1	284.3	49.7
Unamortized Purchased Gas Adjustments | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities	0.0	0.0	17.0
Regulatory liabilities, non current	17.1	284.3	49.7
Unamortized Purchased Gas Adjustments | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities		10.2	10.6
Other
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities	3.2	17.8	21.6
Regulatory liabilities, non current	13.5	13.6	41.7
Other | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities	0.0	13.5	17.0
Regulatory liabilities, non current	7.9	8.0	36.1
Other | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities		1.0	1.4
Regulatory liabilities, non current	3.6	3.6	3.7
Deferred Taxes Due to Customers
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	152.1	127.5	133.2
Deferred Taxes Due to Customers | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	134.7	110.2	115.8
Accrued Cost of Removal
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	38.3	36.2	28.7
Accrued Cost of Removal | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	$ 5.7	$ 4.9	$ 0.0