Regulatory Matters - Schedule of Regulatory Assets and Liabilities, Reflected in Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2019	Sep. 30, 2018
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	$ 78.6	$ 72.8
Regulatory assets, non current	767.6	669.8
Total Regulatory Assets	846.2	742.6
Regulatory liabilities, current	60.8	35.7
Regulatory liabilities, non current	399.0	354.6
Total Regulatory Liabilities	459.8	390.3
Pension and Other Postretirement Benefits
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	30.1	30.2
Regulatory assets, non current	416.6	364.9
Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	18.2	8.2
Regulatory assets, non current	9.1	0.0
Other
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	30.3	34.4
Regulatory assets, non current	47.2	42.4
Future Income Taxes Due from Customers
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	108.8	96.3
Accrued Cost of Removal
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	150.9	133.4
Energy Efficiency
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	35.0	32.8
Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	29.4	30.7
Regulatory assets, non current	507.5	441.1
Total Regulatory Assets	536.9	471.8
Regulatory liabilities, current	52.3	16.7
Regulatory liabilities, non current	326.5	274.9
Total Regulatory Liabilities	378.8	291.6
Spire Missouri | Pension and Other Postretirement Benefits
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	21.9	21.9
Regulatory assets, non current	333.3	292.5
Spire Missouri | Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	0.0	1.0
Regulatory assets, non current	9.1	0.0
Spire Missouri | Other
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	7.5	7.8
Regulatory assets, non current	27.2	21.4
Spire Missouri | Future Income Taxes Due from Customers
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	102.9	94.4
Spire Missouri | Accrued Cost of Removal
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	0.0	0.0
Spire Missouri | Energy Efficiency
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	35.0	32.8
Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	33.9	26.2
Regulatory assets, non current	231.2	201.5
Total Regulatory Assets	265.1	227.7
Regulatory liabilities, current	3.4	7.6
Regulatory liabilities, non current	23.0	31.3
Total Regulatory Liabilities	26.4	38.9
Spire Alabama | Pension and Other Postretirement Benefits
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	7.3	7.3
Regulatory assets, non current	77.2	64.8
Spire Alabama | Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	17.7	6.4
Regulatory assets, non current	0.0	0.0
Spire Alabama | Other
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	8.9	12.5
Regulatory assets, non current	3.1	3.3
Spire Alabama | Future Income Taxes Due from Customers
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	0.0	0.0
Spire Alabama | Accrued Cost of Removal
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	150.9	133.4
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	0.0
Regulatory liabilities, non current	142.3	27.8
Pension and Other Postretirement Benefits | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	3.6	0.0
Regulatory liabilities, non current	119.1	0.0
Pension and Other Postretirement Benefits | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	2.2	0.0
Regulatory liabilities, non current	19.1	27.8
Refundable Negative Salvage
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	0.0	2.2
Refundable Negative Salvage | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	0.0	0.0
Refundable Negative Salvage | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	0.0	2.2
Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	26.2	2.9
Regulatory liabilities, non current	0.0	4.7
Unamortized Purchased Gas Adjustments | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	25.4	1.9
Regulatory liabilities, non current	0.0	4.7
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	28.8	30.6
Regulatory liabilities, non current	35.3	80.2
Other | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	23.3	14.8
Regulatory liabilities, non current	29.2	69.3
Other | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	1.2	5.4
Regulatory liabilities, non current	3.9	3.5
Deferred Taxes Due to Customers
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	179.8	178.3
Deferred Taxes Due to Customers | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	162.5	161.1
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	41.6	63.6
Accrued Cost of Removal | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	15.7	39.8
Accrued Cost of Removal | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	$ 0.0	$ 0.0