Regulatory Matters - Schedule of Regulatory Assets and Liabilities, Reflected in Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	$ 70.5	$ 69.5	$ 67.9
Regulatory assets, non current	1,079.2	1,069.4	750.5
Total Regulatory Assets	1,149.7	1,138.9	818.4
Regulatory liabilities, current	95.5	113.0	50.0
Regulatory liabilities, non current	357.3	343.7	417.8
Total Regulatory Liabilities	452.8	456.7	467.8
Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	32.1	32.1	29.4
Regulatory assets, non current	550.4	548.7	491.1
Total Regulatory Assets	582.5	580.8	520.5
Regulatory liabilities, current	83.4	103.2	42.4
Regulatory liabilities, non current	288.2	274.8	345.5
Total Regulatory Liabilities	371.6	378.0	387.9
Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	22.7	20.4	24.4
Regulatory assets, non current	498.2	489.9	230.2
Total Regulatory Assets	520.9	510.3	254.6
Regulatory liabilities, current	6.9	3.9	2.4
Regulatory liabilities, non current	18.4	18.5	22.3
Total Regulatory Liabilities	25.3	22.4	24.7
Pension and Other Postretirement Benefits
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	30.6	30.6	30.1
Regulatory assets, non current	428.6	439.3	405.5
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	324.7
Pension and Other Postretirement Benefits | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	7.7	7.7	7.2
Regulatory assets, non current	95.7	98.2	74.9
Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	8.4	5.5	9.1
Regulatory assets, non current	3.8	12.1	0.0
Unamortized Purchased Gas Adjustments | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	3.8	12.1	0.0
Unamortized Purchased Gas Adjustments | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	8.3	5.5	8.8
Other
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	31.5	33.4	28.7
Regulatory assets, non current	62.6	59.3	44.1
Other | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	10.2	10.2	7.5
Regulatory assets, non current	45.7	42.7	24.3
Other | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	6.7	7.2	8.4
Regulatory assets, non current	1.2	0.9	3.2
Future Income Taxes Due from Customers
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	125.4	123.5	111.6
Future Income Taxes Due from Customers | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	116.6	114.6	104.9
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	417.4	395.6	152.1
Accrued Cost of Removal | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	18.3	7.1	0.0
Accrued Cost of Removal | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	399.1	388.6	152.1
Energy Efficiency
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	41.4	39.6	37.2
Energy Efficiency | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	41.4	39.6	37.2
Pension and Other Postretirement Benefits
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	5.8	5.8	5.8
Regulatory liabilities, non current	163.7	157.6	147.5
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	146.1	140.4	124.9
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	14.7	14.8	18.4
Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	54.8	73.1	19.9
Regulatory liabilities, non current	14.8	4.4	24.3
Unamortized Purchased Gas Adjustments | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	54.3	72.3	19.0
Regulatory liabilities, non current	14.8	4.4	24.3
Other
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	34.9	34.1	24.3
Regulatory liabilities, non current	14.2	14.3	30.3
Other | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	25.5	27.3	19.8
Regulatory liabilities, non current	8.7	8.6	24.5
Other | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	4.7	1.7	0.2
Regulatory liabilities, non current	3.7	3.7	3.9
Deferred Taxes Due to Customers
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	136.0	138.8	177.0
Deferred Taxes Due to Customers | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	118.6	121.4	159.7
Accrued Cost of Removal
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	28.6	28.6	38.7
Accrued Cost of Removal | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	$ 0.0	$ 0.0	$ 12.1