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Pension Plans and Other Postretirement Benefits - Additional Information (Details) | 3 Months Ended | 6 Months Ended |  |
--- | --- | --- | --- | ---
Mar. 31, 2021 USD ($) plan | Mar. 31, 2020 USD ($) plan | Mar. 31, 2021 USD ($) plan | Sep. 30, 2020 | Sep. 30, 2019
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Medical insurance available until age is reached after early retirement |  |  | 65 years |  |
Other Postretirement Benefits Plan |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Employer contributions |  |  | $ 4,500,000 |  |
Other Postretirement Benefits Plan | Spire Alabama |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Number of Plans | plan | 1 |  | 1 |  |
Settlement benefits paid | $ 12,900,000 |  | $ 0 |  |
Settlement loss | $ 3,800,000 |  |  |  |
Discount rates |  |  |  | 2.95% | 3.20%
Settlement losses amortized over remaining actuarial life |  |  | 11 years 4 months 24 days |  |
Anticipated contributions to pension plans for qualified trust |  |  | $ 6,600,000 |  |
Other Postretirement Benefits Plan | Spire Missouri |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Number of Plans | plan |  | 2 |  |  |
Settlement benefits paid |  | $ 59,100,000 |  |  |
Settlement loss |  | $ 21,000,000.0 |  |  |
Discount rates |  |  |  | 2.85% | 3.20%
Anticipated contributions to pension plans for qualified trust |  |  | 13,700,000 |  |
Other Postretirement Benefits Plan | Spire Missouri | Qualified Plan |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Employer contributions |  |  | 28,300,000 |  |
Other Postretirement Benefits Plan | Spire Missouri | Nonqualified Plan |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Employer contributions |  |  | 0 |  |
Other Postretirement Benefits Plan | Spire Alabama |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Employer contributions |  |  | 0 |  |
Anticipated contributions to pension plans for qualified trust |  |  | 0 |  |
Other Postretirement Benefits Plan | Spire Missouri |  |  |  |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Employer contributions |  |  | 0 |  |
Anticipated contributions to pension plans for qualified trust |  |  | $ 0 |  |

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- Definition Anticipated contributions to the pension plans for the next fiscal year for qualified trusts. + References No definition available. + Details
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- Definition Defined benefit plan benefit obligation settlement losses amortized over remaining actuarial life. + References No definition available. + Details
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- Definition Reflects the upper limit on age eligibility, post early retirement, when medical insurance is available to retiree. + References No definition available. + Details
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- Definition Number of plans. + References No definition available. + Details
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- Definition Weighted average rate increase of compensation, used to determine benefit obligation of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(2) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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- Definition Amount of payment, which decreases benefit obligation of defined benefit plan, for irrevocable action relieving primary responsibility for benefit obligation and eliminating risk for obligation and assets used to effect settlement. Transaction constituting settlement includes, but is not limited to, making lump-sum cash payment to participant in exchange for their rights to receive specified benefits and purchasing nonparticipating annuity contract. Excludes decreases to benefit obligation for remeasurement due to settlement. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(10) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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