Regulatory Matters - Schedule of Regulatory Assets and Liabilities, Reflected in Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2021	Sep. 30, 2021	Dec. 31, 2020
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	$ 265.7	$ 306.5	$ 70.5
Regulatory assets, non current	1,089.9	993.5	1,079.2
Total Regulatory Assets	1,355.6	1,300.0	1,149.7
Regulatory liabilities, current	4.1	34.6	95.5
Regulatory liabilities, non current	531.8	620.9	357.3
Total Regulatory Liabilities	535.9	655.5	452.8
Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	236.9	276.3	32.1
Regulatory assets, non current	560.5	483.1	550.4
Total Regulatory Assets	797.4	759.4	582.5
Regulatory liabilities, current	0.2	17.1	83.4
Regulatory liabilities, non current	447.2	538.8	288.2
Total Regulatory Liabilities	447.4	555.9	371.6
Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	14.6	18.8	22.7
Regulatory assets, non current	501.7	483.3	498.2
Total Regulatory Assets	516.3	502.1	520.9
Regulatory liabilities, current	0.8	13.4	6.9
Regulatory liabilities, non current	25.3	23.4	18.4
Total Regulatory Liabilities	26.1	36.8	25.3
Pension and Other Postretirement Benefits
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	0.0	31.1	30.6
Regulatory assets, non current	335.9	313.8	428.6
Pension and Other Postretirement Benefits | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	0.0	21.9	21.9
Regulatory assets, non current	241.6	226.0	324.6
Pension and Other Postretirement Benefits | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current		8.2	7.7
Regulatory assets, non current	88.7	82.9	95.7
Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	242.4	243.5	8.4
Regulatory assets, non current	27.3	0.0	3.8
Unamortized Purchased Gas Adjustments | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	235.2	242.8	0.0
Regulatory assets, non current	27.3	0.0	3.8
Unamortized Purchased Gas Adjustments | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	4.1	0.0	8.3
Other
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	23.3	31.9	31.5
Regulatory assets, non current	94.7	67.3	62.6
Other | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	1.7	11.6	10.2
Regulatory assets, non current	77.8	50.4	45.7
Other | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	10.5	10.6	6.7
Regulatory assets, non current	1.2	1.2	1.2
Future Income Taxes Due from Customers
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	135.5	132.9	125.4
Future Income Taxes Due from Customers | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	126.9	124.2	116.6
Future Income Taxes Due from Customers | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	2.2	2.2	2.2
Accrued Cost of Removal
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	444.5	431.9	417.4
Accrued Cost of Removal | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	34.9	34.9	18.3
Accrued Cost of Removal | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	409.6	397.0	399.1
Energy Efficiency
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	52.0	47.6	41.4
Energy Efficiency | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	52.0	47.6	41.4
Pension and Other Postretirement Benefits
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	0.0	5.8	5.8
Regulatory liabilities, non current	172.2	159.3	163.7
Pension and Other Postretirement Benefits | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	0.0	3.6	3.6
Regulatory liabilities, non current	142.6	131.4	146.1
Pension and Other Postretirement Benefits | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current		2.2	2.2
Regulatory liabilities, non current	21.6	19.8	14.7
Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	0.0	11.0	54.8
Regulatory liabilities, non current	155.0	284.3	14.8
Unamortized Purchased Gas Adjustments | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	0.0	0.0	54.3
Regulatory liabilities, non current	155.0	284.3	14.8
Unamortized Purchased Gas Adjustments | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current		10.2	0.0
Other
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	4.1	17.8	34.9
Regulatory liabilities, non current	13.8	13.6	14.2
Other | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	0.2	13.5	25.5
Regulatory liabilities, non current	8.1	8.0	8.7
Other | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	0.8	1.0	4.7
Regulatory liabilities, non current	3.7	3.6	3.7
Deferred Taxes Due to Customers
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	155.5	127.5	136.0
Deferred Taxes Due to Customers | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	138.2	110.2	118.6
Accrued Cost of Removal
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	35.3	36.2	28.6
Accrued Cost of Removal | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	$ 3.3	$ 4.9	$ 0.0