Regulatory Matters - Schedule of Regulatory Assets and Liabilities, Reflected in Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020
Regulatory Asset And Liabilities [Line Items]
Regulatory assets	$ 306.5	$ 69.5
Regulatory assets, non current	993.5	1,069.4
Total Regulatory Assets	1,300.0	1,138.9
Regulatory liabilities	34.6	113.0
Regulatory liabilities, non current	620.9	343.7
Total Regulatory Liabilities	655.5	456.7
Pension and Other Postretirement Benefits
Regulatory Asset And Liabilities [Line Items]
Regulatory assets	31.1	30.6
Regulatory assets, non current	313.8	439.3
Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory assets	243.5	5.5
Regulatory assets, non current	0.0	12.1
Other
Regulatory Asset And Liabilities [Line Items]
Regulatory assets	31.9	33.4
Regulatory assets, non current	67.3	59.3
Future Income Taxes Due from Customers
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	132.9	123.5
Accrued Cost of Removal
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	431.9	395.6
Energy Efficiency
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	47.6	39.6
Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets	276.3	32.1
Regulatory assets, non current	483.1	548.7
Total Regulatory Assets	759.4	580.8
Regulatory liabilities	17.1	103.2
Regulatory liabilities, non current	538.8	274.8
Total Regulatory Liabilities	555.9	378.0
Spire Missouri | Pension and Other Postretirement Benefits
Regulatory Asset And Liabilities [Line Items]
Regulatory assets	21.9	21.9
Regulatory assets, non current	226.0	332.6
Spire Missouri | Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory assets	242.8	0.0
Regulatory assets, non current	0.0	12.1
Spire Missouri | Other
Regulatory Asset And Liabilities [Line Items]
Regulatory assets	11.6	10.2
Regulatory assets, non current	50.4	42.7
Spire Missouri | Future Income Taxes Due from Customers
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	124.2	114.6
Spire Missouri | Accrued Cost of Removal
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	34.9	7.1
Spire Missouri | Energy Efficiency
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	47.6	39.6
Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets	18.8	20.4
Regulatory assets, non current	483.3	489.9
Total Regulatory Assets	502.1	510.3
Regulatory liabilities	13.4	3.9
Regulatory liabilities, non current	23.4	18.5
Total Regulatory Liabilities	36.8	22.4
Spire Alabama | Pension and Other Postretirement Benefits
Regulatory Asset And Liabilities [Line Items]
Regulatory assets	8.2	7.7
Regulatory assets, non current	82.9	98.2
Spire Alabama | Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory assets	0.0	5.5
Regulatory assets, non current	0.0	0.0
Spire Alabama | Other
Regulatory Asset And Liabilities [Line Items]
Regulatory assets	10.6	7.2
Regulatory assets, non current	1.2	0.9
Spire Alabama | Future Income Taxes Due from Customers
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	2.2	2.2
Spire Alabama | Accrued Cost of Removal
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	397.0	388.6
Spire Alabama | Energy Efficiency
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	0.0	0.0
Pension and Other Postretirement Benefits
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities	5.8	5.8
Regulatory liabilities, non current	159.3	157.6
Pension and Other Postretirement Benefits | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities	3.6	3.6
Regulatory liabilities, non current	131.4	140.4
Pension and Other Postretirement Benefits | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities	2.2	2.2
Regulatory liabilities, non current	19.8	14.8
Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities	11.0	73.1
Regulatory liabilities, non current	284.3	4.4
Unamortized Purchased Gas Adjustments | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities	0.0	72.3
Regulatory liabilities, non current	284.3	4.4
Unamortized Purchased Gas Adjustments | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities	10.2	0.0
Regulatory liabilities, non current	0.0	0.0
Other
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities	17.8	34.1
Regulatory liabilities, non current	13.6	14.3
Other | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities	13.5	27.3
Regulatory liabilities, non current	8.0	8.6
Other | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities	1.0	1.7
Regulatory liabilities, non current	3.6	3.7
Deferred Taxes Due to Customers
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	127.5	138.8
Deferred Taxes Due to Customers | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	110.2	121.4
Deferred Taxes Due to Customers | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	0.0	0.0
Accrued Cost of Removal
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	36.2	28.6
Accrued Cost of Removal | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	4.9	0.0
Accrued Cost of Removal | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	$ 0.0	$ 0.0