Regulatory Matters - Schedule of Regulatory Assets and Liabilities, Reflected in Balance Sheets (Details) - USD ($) $ in Millions	Jun. 30, 2020	Sep. 30, 2019	Jun. 30, 2019
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	$ 70.2	$ 78.6	$ 79.0
Regulatory assets, non current	791.8	767.6	657.1
Total Regulatory Assets	862.0	846.2	736.1
Regulatory liabilities, current	58.8	60.8	31.3
Regulatory liabilities, non current	449.6	399.0	396.3
Total Regulatory Liabilities	508.4	459.8	427.6
Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	29.5	29.4	30.2
Regulatory assets, non current	531.9	507.5	429.2
Total Regulatory Assets	561.4	536.9	459.4
Regulatory liabilities, current	50.8	52.3	23.4
Regulatory liabilities, non current	378.9	326.5	310.5
Total Regulatory Liabilities	429.7	378.8	333.9
Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	23.4	33.9	33.9
Regulatory assets, non current	232.1	231.2	199.9
Total Regulatory Assets	255.5	265.1	233.8
Regulatory liabilities, current	2.5	3.4	3.4
Regulatory liabilities, non current	20.9	23.0	29.8
Total Regulatory Liabilities	23.4	26.4	33.2
Pension and Other Postretirement Benefits
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	30.1	30.1	30.1
Regulatory assets, non current	415.3	416.6	335.7
Pension and Other Postretirement Benefits | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	21.9	21.9	21.9
Regulatory assets, non current	340.6	333.3	269.8
Pension and Other Postretirement Benefits | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	7.2	7.3	7.3
Regulatory assets, non current	69.9	77.2	59.3
Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	9.6	18.2	18.9
Regulatory assets, non current	0.0	9.1	0.0
Unamortized Purchased Gas Adjustments | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	0.1	0.0	0.8
Regulatory assets, non current	0.0	9.1	0.0
Unamortized Purchased Gas Adjustments | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	9.5	17.7	17.6
Other
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	30.5	30.3	30.0
Regulatory assets, non current	57.2	47.2	45.7
Other | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	7.5	7.5	7.5
Regulatory assets, non current	40.0	27.2	25.5
Other | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	6.7	8.9	9.0
Regulatory assets, non current	0.9	3.1	3.2
Future Income Taxes Due from Customers
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	120.7	108.8	105.7
Future Income Taxes Due from Customers | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	111.8	102.9	101.3
Future Income Taxes Due from Customers | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	2.2	0.0	0.0
Accrued Cost of Removal
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	160.4	150.9	137.4
Accrued Cost of Removal | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	1.3	0.0	0.0
Accrued Cost of Removal | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	159.1	150.9	137.4
Energy Efficiency
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	38.2	35.0	32.6
Energy Efficiency | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	38.2	35.0	32.6
Pension and Other Postretirement Benefits
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	5.8	5.8	5.8
Regulatory liabilities, non current	157.6	142.3	119.8
Pension and Other Postretirement Benefits | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	3.6	3.6	3.6
Regulatory liabilities, non current	136.4	119.1	84.6
Pension and Other Postretirement Benefits | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	2.2	2.2	2.3
Regulatory liabilities, non current	17.2	19.1	25.8
Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	21.1	26.2	3.3
Regulatory liabilities, non current	79.9	0.0	47.9
Unamortized Purchased Gas Adjustments | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	20.0	25.4	2.5
Regulatory liabilities, non current	79.9	0.0	47.9
Other
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	31.9	28.8	22.2
Regulatory liabilities, non current	29.3	35.3	29.9
Other | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	27.2	23.3	17.3
Regulatory liabilities, non current	23.6	29.2	24.1
Other | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	0.3	1.2	1.1
Regulatory liabilities, non current	3.7	3.9	4.0
Deferred Taxes Due to Customers
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	156.4	179.8	152.9
Deferred Taxes Due to Customers | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	139.0	162.5	135.5
Accrued Cost of Removal
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	26.4	41.6	45.8
Accrued Cost of Removal | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	$ 0.0	$ 15.7	$ 18.4