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Pension Plans and Other Postretirement Benefits - Additional Information (Details) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Sep. 30, 2020 USD ($) plan | Sep. 30, 2019 USD ($) plan | Sep. 30, 2018 USD ($) | Apr. 19, 2018 USD ($) | Apr. 18, 2018 USD ($)
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Implementation period used to calculate return on plan assets | 4 years |  |  |  |
Gains or losses includible in cost amortized to extent exceeds benefit obligations or market-related value of plan assets, percentage | 10.00% |  |  |  |
Regulatory assets | $ 1,138.9 | $ 846.2 |  |  |
Remaining recovery period for regulatory assets for which no return on investment during recovery period is provided | 15 years |  |  |  |
Medical insurance available until age is reached after early retirement | 65 years |  |  |  |
Recovery in rates for postretirement benefit |  |  |  |  | $ 8.6
401(K) plans, cost of defined contributions | $ 13.6 | 13.8 | $ 12.5 |  |
Spire Missouri |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Regulatory assets | 580.8 | 536.9 |  |  |
401(K) plans, cost of defined contributions | 9.5 | 10.0 | 9.0 |  |
Spire Alabama |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Regulatory assets | 510.3 | 265.1 |  |  |
401(K) plans, cost of defined contributions | 3.4 | $ 3.1 | $ 3.0 |  |
Spire Missouri West | Pension Cost | MoPSC | Infrastructure System Replacement Surcharge |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Regulatory assets |  |  |  | $ 5.5 | 9.9
Spire Missouri West | Pension and Other Postretirement Benefits |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Amortization of assets | $ 26.2 |  |  |  |
Spire Missouri East | Pension Cost | MoPSC | Infrastructure System Replacement Surcharge |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Regulatory assets |  |  |  | $ 29.0 | $ 15.5
Spire Missouri East | Pension and Other Postretirement Benefits |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Remaining recovery period for regulatory assets for which no return on investment during recovery period is provided | 8 years |  |  |  |
Amortization of assets | $ 173.0 |  |  |  |
Pension Plans |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Discount rates | 3.00% | 3.00% |  |  |
Settlement benefits paid | $ 89.3 |  |  |  |
Employer contributions | $ 43.7 | $ 27.8 |  |  |
Pension Plans | Spire Missouri |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Discount rates | 2.85% | 3.20% |  |  |
Settlement benefits paid | $ 74.5 |  |  |  |
Number Of Plans | plan | 2 |  |  |  |
Anticipated Contributions to pension plans for qualified trust | $ 36.4 |  |  |  |
Anticipated contributions to pension plans for non-qualified plans | 0.6 |  |  |  |
Employer contributions | $ 30.3 | $ 27.1 |  |  |
Pension Plans | Spire Alabama |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Discount rates | 2.95% | 3.20% |  |  |
Settlement benefits paid | $ 14.8 |  |  |  |
Number Of Plans | plan | 1 | 0 |  |  |
Anticipated Contributions to pension plans for qualified trust | $ 11.3 |  |  |  |
Employer contributions | 12.7 | $ 0.5 |  |  |
Other Postretirement Benefits Plan |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Employer contributions | 0.2 | 0.6 |  |  |
Anticipated contributions to the postretirement plans for qualified trusts | 4.8 |  |  |  |
Anticipated payment directly to participants in postretirement plan | 0.6 |  |  |  |
Other Postretirement Benefits Plan | Spire Missouri |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Employer contributions | $ 0.2 | $ 0.6 |  |  |
Other Postretirement Benefits Plan | Spire Missouri East |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Discount rates | 3.00% | 3.00% |  |  |

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