Regulatory Matters - Schedule of Regulatory Assets and Liabilities, Reflected in Balance Sheets (Details) - USD ($) $ in Millions	Mar. 31, 2020	Sep. 30, 2019	Mar. 31, 2019
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	$ 68.0	$ 78.6	$ 75.3
Regulatory assets, non current	770.7	767.6	659.9
Total Regulatory Assets	838.7	846.2	735.2
Regulatory liabilities, current	31.8	60.8	35.3
Regulatory liabilities, non current	472.3	399.0	431.3
Total Regulatory Liabilities	504.1	459.8	466.6
Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	29.4	29.4	30.0
Regulatory assets, non current	512.1	507.5	433.6
Total Regulatory Assets	541.5	536.9	463.6
Regulatory liabilities, current	24.3	52.3	24.2
Regulatory liabilities, non current	400.4	326.5	345.1
Total Regulatory Liabilities	424.7	378.8	369.3
Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	23.3	33.9	30.4
Regulatory assets, non current	230.1	231.2	198.9
Total Regulatory Assets	253.4	265.1	229.3
Regulatory liabilities, current	2.4	3.4	6.1
Regulatory liabilities, non current	21.6	23.0	30.5
Total Regulatory Liabilities	24.0	26.4	36.6
Pension and Other Postretirement Benefits
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	30.1	30.1	30.2
Regulatory assets, non current	415.6	416.6	345.8
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	337.5	333.3	277.9
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	72.6	77.2	61.1
Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	10.4	18.2	19.3
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	0.0	0.0	0.6
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	10.4	17.7	18.0
Other
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	27.5	30.3	25.8
Regulatory assets, non current	49.6	47.2	45.3
Other | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	7.5	7.5	7.5
Regulatory assets, non current	30.1	27.2	24.7
Other | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	5.7	8.9	5.1
Regulatory assets, non current	3.2	3.1	3.2
Future Income Taxes Due from Customers
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	113.5	108.8	102.2
Future Income Taxes Due from Customers | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	106.8	102.9	99.0
Accrued Cost of Removal
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	154.3	150.9	134.6
Accrued Cost of Removal | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	154.3	150.9	134.6
Energy Efficiency
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	37.7	35.0	32.0
Energy Efficiency | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	37.7	35.0	32.0
Pension and Other Postretirement Benefits
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	5.8	5.8	5.8
Regulatory liabilities, non current	152.5	142.3	115.3
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	130.7	119.1	79.4
Pension and Other Postretirement Benefits | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	2.2	2.2	2.2
Regulatory liabilities, non current	17.8	19.1	26.5
Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	8.0	26.2	3.4
Regulatory liabilities, non current	82.6	0.0	75.7
Unamortized Purchased Gas Adjustments | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	7.1	25.4	2.7
Regulatory liabilities, non current	82.6	0.0	75.7
Other
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	18.0	28.8	26.1
Regulatory liabilities, non current	31.3	35.3	30.9
Other | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	13.6	23.3	17.9
Regulatory liabilities, non current	25.5	29.2	24.9
Other | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	0.2	1.2	3.9
Regulatory liabilities, non current	3.8	3.9	4.0
Deferred Taxes Due to Customers
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	174.2	179.8	155.7
Deferred Taxes Due to Customers | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	156.8	162.5	138.3
Accrued Cost of Removal
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	31.7	41.6	53.7
Accrued Cost of Removal | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	$ 4.8	$ 15.7	$ 26.8