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 | 60.8 | 37 |
Regulatory liabilities, non current | 417.8 | 399 | 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 |
Regulatory liabilities, current | 2.4 | 3.4 | 10.2 |
Regulatory liabilities, non current | 22.3 | 23 | 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 |
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 |
Unamortized Purchased Gas Adjustments | | | |
Regulatory Asset And Liabilities [Line Items] | | | |
Regulatory assets, current | 9.1 | 18.2 | 3 |
Regulatory assets, non current | 0 | 9.1 | 0 |
Unamortized Purchased Gas Adjustments | Spire Missouri | | | |
Regulatory Asset And Liabilities [Line Items] | | | |
Regulatory assets, non current | 0 | 9.1 | 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 |
Accrued Cost of Removal | Spire Alabama | | | |
Regulatory Asset And Liabilities [Line Items] | | | |
Regulatory assets, non current | 152.1 | 150.9 | 134 |
Energy Efficiency | | | |
Regulatory Asset And Liabilities [Line Items] | | | |
Regulatory assets, non current | 37.2 | 35 | 33.6 |
Energy Efficiency | Spire Missouri | | | |
Regulatory Asset And Liabilities [Line Items] | | | |
Regulatory assets, non current | 37.2 | 35 | 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 |
Regulatory liabilities, non current | 124.9 | 119.1 | 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 | 16.7 |
Unamortized Purchased Gas Adjustments | Spire Missouri | | | |
Regulatory Asset And Liabilities [Line Items] | | | |
Regulatory liabilities, current | 19 | 25.4 | 5.8 |
Regulatory liabilities, non current | 24.3 | 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 | 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 |