Regulatory Matters - Schedule of Regulatory Assets and Liabilities, Reflected in Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	$ 69.5	$ 78.6
Regulatory assets, non current	1,069.4	767.6
Total Regulatory Assets	1,138.9	846.2
Regulatory liabilities, current	113.0	60.8
Regulatory liabilities, non current	343.7	399.0
Total Regulatory Liabilities	456.7	459.8
Pension and Other Postretirement Benefits
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	30.6	30.1
Regulatory assets, non current	439.3	416.6
Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	5.5	18.2
Regulatory assets, non current	12.1	9.1
Other
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	33.4	30.3
Regulatory assets, non current	59.3	45.0
Future Income Taxes Due from Customers
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	123.5	111.0
Accrued Cost of Removal
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	395.6	150.9
Energy Efficiency
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	39.6	35.0
Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	32.1	29.4
Regulatory assets, non current	548.7	507.5
Total Regulatory Assets	580.8	536.9
Regulatory liabilities, current	103.2	52.3
Regulatory liabilities, non current	274.8	326.5
Total Regulatory Liabilities	378.0	378.8
Spire Missouri | Pension and Other Postretirement Benefits
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	21.9	21.9
Regulatory assets, non current	332.6	333.3
Spire Missouri | Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	0.0	0.0
Regulatory assets, non current	12.1	9.1
Spire Missouri | Other
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	10.2	7.5
Regulatory assets, non current	42.7	27.2
Spire Missouri | Future Income Taxes Due from Customers
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	114.6	102.9
Spire Missouri | Accrued Cost of Removal
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	7.1	0.0
Spire Missouri | Energy Efficiency
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	39.6	35.0
Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	20.4	33.9
Regulatory assets, non current	489.9	231.2
Total Regulatory Assets	510.3	265.1
Regulatory liabilities, current	3.9	3.4
Regulatory liabilities, non current	18.5	23.0
Total Regulatory Liabilities	22.4	26.4
Spire Alabama | Pension and Other Postretirement Benefits
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	7.7	7.3
Regulatory assets, non current	98.2	77.2
Spire Alabama | Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	5.5	17.7
Regulatory assets, non current	0.0	0.0
Spire Alabama | Other
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	7.2	8.9
Regulatory assets, non current	0.9	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	388.6	150.9
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, current	5.8	5.8
Regulatory liabilities, non current	157.6	142.3
Pension and Other Postretirement Benefits | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	3.6	3.6
Regulatory liabilities, non current	140.4	119.1
Pension and Other Postretirement Benefits | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	2.2	2.2
Regulatory liabilities, non current	14.8	19.1
Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	73.1	26.2
Regulatory liabilities, non current	4.4	0.0
Unamortized Purchased Gas Adjustments | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	72.3	25.4
Regulatory liabilities, non current	4.4	0.0
Unamortized Purchased Gas Adjustments | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	0.0	0.0
Regulatory liabilities, non current	0.0	0.0
Other
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	34.1	28.8
Regulatory liabilities, non current	14.3	35.3
Other | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	27.3	23.3
Regulatory liabilities, non current	8.6	29.2
Other | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	1.7	1.2
Regulatory liabilities, non current	3.7	3.9
Deferred Taxes Due to Customers
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	138.8	179.8
Deferred Taxes Due to Customers | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	121.4	162.5
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	28.6	41.6
Accrued Cost of Removal | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	0.0	15.7
Accrued Cost of Removal | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	$ 0.0	$ 0.0