Regulatory Matters - Schedule of Regulatory Assets and Liabilities, Reflected in Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	$ 67.9	$ 78.6	$ 63.4
Regulatory assets, non current	750.5	767.6	655.1
Total Regulatory Assets	818.4	846.2	718.5
Regulatory liabilities, current	50.0	60.8	37.0
Regulatory liabilities, non current	417.8	399.0	363.4
Total Regulatory Liabilities	467.8	459.8	400.4
Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	29.4	29.4	29.7
Regulatory assets, non current	491.1	507.5	427.6
Total Regulatory Assets	520.5	536.9	457.3
Regulatory liabilities, current	42.4	52.3	20.6
Regulatory liabilities, non current	345.5	326.5	279.7
Total Regulatory Liabilities	387.9	378.8	300.3
Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	24.4	33.9	18.8
Regulatory assets, non current	230.2	231.2	200.2
Total Regulatory Assets	254.6	265.1	219.0
Regulatory liabilities, current	2.4	3.4	10.2
Regulatory liabilities, non current	22.3	23.0	30.6
Total Regulatory Liabilities	24.7	26.4	40.8
Pension and Other Postretirement Benefits
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	30.1	30.1	30.2
Regulatory assets, non current	405.5	416.6	350.0
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	324.7	333.3	279.8
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	74.9	77.2	63.0
Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	9.1	18.2	3.0
Regulatory assets, non current	0.0	9.1	0.0
Unamortized Purchased Gas Adjustments | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
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	8.8	17.7	1.6
Other
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	28.7	30.3	30.2
Regulatory assets, non current	44.1	47.2	38.4
Other | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	7.5	7.5	7.8
Regulatory assets, non current	24.3	27.2	17.5
Other | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, current	8.4	8.9	9.9
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	111.6	108.8	99.1
Future Income Taxes Due from Customers | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	104.9	102.9	96.7
Accrued Cost of Removal
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	152.1	150.9	134.0
Accrued Cost of Removal | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	152.1	150.9	134.0
Energy Efficiency
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	37.2	35.0	33.6
Energy Efficiency | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory assets, non current	37.2	35.0	33.6
Pension and Other Postretirement Benefits
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	5.8	5.8	2.2
Regulatory liabilities, non current	147.5	142.3	27.1
Pension and Other Postretirement Benefits | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	3.6	3.6	0.0
Regulatory liabilities, non current	124.9	119.1	0.0
Pension and Other Postretirement Benefits | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	2.2	2.2	2.3
Regulatory liabilities, non current	18.4	19.1	27.1
Unamortized Purchased Gas Adjustments
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	19.9	26.2	6.1
Regulatory liabilities, non current	24.3	0.0	16.7
Unamortized Purchased Gas Adjustments | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	19.0	25.4	5.8
Regulatory liabilities, non current	24.3	0.0	16.7
Other
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	24.3	28.8	28.7
Regulatory liabilities, non current	30.3	35.3	90.4
Other | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	19.8	23.3	14.8
Regulatory liabilities, non current	24.5	29.2	75.4
Other | Spire Alabama
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, current	0.2	1.2	7.9
Regulatory liabilities, non current	3.9	3.9	3.5
Deferred Taxes Due to Customers
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	177.0	179.8	169.8
Deferred Taxes Due to Customers | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	159.7	162.5	152.4
Accrued Cost of Removal
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	38.7	41.6	59.4
Accrued Cost of Removal | Spire Missouri
Regulatory Asset And Liabilities [Line Items]
Regulatory liabilities, non current	$ 12.1	$ 15.7	$ 35.2