Regulatory Matters - Schedule of Regulatory Assets and Liabilities, Reflected in Balance Sheets (Details) - USD ($) $ in Millions	Jun. 30, 2021	Sep. 30, 2020	Jun. 30, 2020
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	$ 65.1	$ 69.5	$ 70.2
Regulatory assets, non current	1,119.4	1,069.4	791.8
Total Regulatory Assets	1,184.5	1,138.9	862.0
Regulatory liabilities, current	46.8	113.0	58.8
Regulatory liabilities, non current	414.6	343.7	449.6
Total Regulatory Liabilities	461.4	456.7	508.4
Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	36.6	32.1	29.5
Regulatory assets, non current	583.8	548.7	531.9
Total Regulatory Assets	620.4	580.8	561.4
Regulatory liabilities, current	27.1	103.2	50.8
Regulatory liabilities, non current	342.8	274.8	378.9
Total Regulatory Liabilities	369.9	378.0	429.7
Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	17.1	20.4	23.4
Regulatory assets, non current	508.2	489.9	232.1
Total Regulatory Assets	525.3	510.3	255.5
Regulatory liabilities, current	15.5	3.9	2.5
Regulatory liabilities, non current	17.5	18.5	20.9
Total Regulatory Liabilities	33.0	22.4	23.4
Pension and Other Postretirement Benefits
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	31.0	30.6	30.1
Regulatory assets, non current	377.3	439.3	415.3
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	287.3	332.6	340.6
Pension and Other Postretirement Benefits | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	8.2	7.7	7.2
Regulatory assets, non current	85.1	98.2	69.9
Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	0.5	5.5	9.6
Regulatory assets, non current	43.9	12.1	0.0
Unamortized Purchased Gas Adjustments | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	0.0	0.0	0.1
Regulatory assets, non current	43.9	12.1	0.0
Unamortized Purchased Gas Adjustments | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current		5.5	9.5
Other
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	33.6	33.4	30.5
Regulatory assets, non current	73.3	59.3	57.2
Other | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	14.7	10.2	7.5
Regulatory assets, non current	56.1	42.7	40.0
Other | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	8.9	7.2	6.7
Regulatory assets, non current	1.2	0.9	0.9
Future Income Taxes Due from Customers
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	130.2	123.5	120.7
Future Income Taxes Due from Customers | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	121.5	114.6	111.8
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	449.1	395.6	160.4
Accrued Cost of Removal | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	29.4	7.1	1.3
Accrued Cost of Removal | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	419.7	388.6	159.1
Energy Efficiency
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	45.6	39.6	38.2
Energy Efficiency | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	45.6	39.6	38.2
Pension and Other Postretirement Benefits
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	5.8	5.8	5.8
Regulatory liabilities, non current	176.7	157.6	157.6
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	157.0	140.4	136.4
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	13.9	14.8	17.2
Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	23.0	73.1	21.1
Regulatory liabilities, non current	42.2	4.4	79.9
Unamortized Purchased Gas Adjustments | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	9.0	72.3	20.0
Regulatory liabilities, non current	42.2	4.4	79.9
Unamortized Purchased Gas Adjustments | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	13.1	0.0	0.0
Other
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	18.0	34.1	31.9
Regulatory liabilities, non current	36.2	14.3	29.3
Other | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	14.5	27.3	27.2
Regulatory liabilities, non current	30.6	8.6	23.6
Other | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	0.2	1.7	0.3
Regulatory liabilities, non current	3.6	3.7	3.7
Deferred Taxes Due to Customers
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	130.4	138.8	156.4
Deferred Taxes Due to Customers | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	113.0	121.4	139.0
Accrued Cost of Removal
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	$ 29.1	$ 28.6	$ 26.4