Regulatory Matters - Schedule of Regulatory Assets and Liabilities, Reflected in Balance Sheets (Details) - USD ($) $ in Millions	Mar. 31, 2021	Sep. 30, 2020	Mar. 31, 2020
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	$ 60.4	$ 69.5	$ 68.0
Regulatory assets, non current	1,133.6	1,069.4	770.7
Total Regulatory Assets	1,194.0	1,138.9	838.7
Regulatory liabilities, current	56.3	113.0	31.8
Regulatory liabilities, non current	423.5	343.7	472.3
Total Regulatory Liabilities	479.8	456.7	504.1
Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	37.5	32.1	29.4
Regulatory assets, non current	604.3	548.7	512.1
Total Regulatory Assets	641.8	580.8	541.5
Regulatory liabilities, current	37.6	103.2	24.3
Regulatory liabilities, non current	353.5	274.8	400.4
Total Regulatory Liabilities	391.1	378.0	424.7
Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	11.8	20.4	23.3
Regulatory assets, non current	501.2	489.9	230.1
Total Regulatory Assets	513.0	510.3	253.4
Regulatory liabilities, current	14.2	3.9	2.4
Regulatory liabilities, non current	18.0	18.5	21.6
Total Regulatory Liabilities	32.2	22.4	24.0
Pension and Other Postretirement Benefits
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	30.9	30.6	30.1
Regulatory assets, non current	418.3	439.3	415.6
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	324.6	332.6	337.5
Pension and Other Postretirement Benefits | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	8.0	7.7	7.2
Regulatory assets, non current	88.5	98.2	72.6
Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	0.1	5.5	10.4
Regulatory assets, non current	51.5	12.1	0.0
Unamortized Purchased Gas Adjustments | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	51.5	12.1	0.0
Unamortized Purchased Gas Adjustments | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current		5.5	10.4
Other
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	29.4	33.4	27.5
Regulatory assets, non current	62.5	59.3	49.6
Other | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	15.6	10.2	7.5
Regulatory assets, non current	45.1	42.7	30.1
Other | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	3.8	7.2	5.7
Regulatory assets, non current	1.1	0.9	3.2
Future Income Taxes Due from Customers
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	127.4	123.5	113.5
Future Income Taxes Due from Customers | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	118.6	114.6	106.8
Future Income Taxes Due from Customers | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	2.2	2.2	0.0
Accrued Cost of Removal
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	430.7	395.6	154.3
Accrued Cost of Removal | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	21.3	7.1	0.0
Accrued Cost of Removal | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	409.4	388.6	154.3
Energy Efficiency
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	43.2	39.6	37.7
Energy Efficiency | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	43.2	39.6	37.7
Pension and Other Postretirement Benefits
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	5.8	5.8	5.8
Regulatory liabilities, non current	170.2	157.6	152.5
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	151.9	140.4	130.7
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	14.3	14.8	17.8
Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	28.9	73.1	8.0
Regulatory liabilities, non current	49.7	4.4	82.6
Unamortized Purchased Gas Adjustments | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	17.0	72.3	7.1
Regulatory liabilities, non current	49.7	4.4	82.6
Unamortized Purchased Gas Adjustments | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	10.6	0.0	0.0
Other
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	21.6	34.1	18.0
Regulatory liabilities, non current	41.7	14.3	31.3
Other | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	17.0	27.3	13.6
Regulatory liabilities, non current	36.1	8.6	25.5
Other | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	1.4	1.7	0.2
Regulatory liabilities, non current	3.7	3.7	3.8
Deferred Taxes Due to Customers
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	133.2	138.8	174.2
Deferred Taxes Due to Customers | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	115.8	121.4	156.8
Accrued Cost of Removal
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	28.7	28.6	31.7
Accrued Cost of Removal | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	$ 0.0	$ 0.0	$ 4.8