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Pension Plans and Other Postretirement Benefits - Reconciliation of the Beginning and Ending Balances of Benefit Obligation (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018
Pension Plans |  |  |
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] |  |  |
Benefit obligation, beginning of year | $ 751.4 | $ 664.6 |
Service cost | 22.5 | 19.3 | $ 20.2
Interest cost | 22.6 | 28.2 | 27.4
Actuarial loss (gain) | 37.5 | 104.8 |
Plan amendments | (4.4) | (10.2) |
Settlement loss | 16.5 |  |
Settlement benefits paid | (89.3) |  |
Regular benefits paid | (24.2) | (55.3) |
Benefit obligation, end of year | 732.6 | 751.4 | 664.6
Accumulated benefit obligation, end of year | 699.3 | 712.9 |
Pension Plans | Spire Missouri |  |  |
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] |  |  |
Benefit obligation, beginning of year | 538.4 | 473.3 |
Service cost | 15.7 | 12.4 | 12.7
Interest cost | 15.8 | 19.8 | 19.5
Actuarial loss (gain) | 15.6 | 76.0 |
Plan amendments | (4.4) | (3.7) |
Settlement loss | 16.5 |  |
Settlement benefits paid | (74.5) |  |
Regular benefits paid | (17.9) | (39.4) |
Benefit obligation, end of year | 505.2 | 538.4 | 473.3
Accumulated benefit obligation, end of year | 473.7 | 500.2 |
Pension Plans | Spire Alabama |  |  |
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] |  |  |
Benefit obligation, beginning of year | 152.5 | 136.3 |
Service cost | 6.1 | 6.2 | 6.4
Interest cost | 4.9 | 6.0 | 5.5
Actuarial loss (gain) | 18.4 | 23.7 |
Plan amendments |  | (6.5) |
Settlement benefits paid | (14.8) |  |
Regular benefits paid | (3.6) | (13.2) |
Benefit obligation, end of year | 163.5 | 152.5 | 136.3
Accumulated benefit obligation, end of year | 161.8 | 152.3 |
Other Postretirement Plans |  |  |
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] |  |  |
Benefit obligation, beginning of year | 197.3 | 208.1 |
Service cost | 5.9 | 7.4 | 9.4
Interest cost | 6.3 | 9.0 | 8.6
Actuarial loss (gain) | 0.2 | (18.9) |
Plan amendments | 15.8 | 5.5 |
Retiree drug subsidy program |  | 0.2 |
Regular benefits paid | (13.2) | (14.0) |
Benefit obligation, end of year | 212.3 | 197.3 | 208.1
Other Postretirement Plans | Spire Missouri |  |  |
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] |  |  |
Benefit obligation, beginning of year | 147.9 | 158.8 |
Service cost | 5.3 | 6.8 | 9.0
Interest cost | 4.7 | 6.9 | 6.9
Actuarial loss (gain) | 1.7 | (19.5) |
Plan amendments | 9.5 | 4.9 |
Retiree drug subsidy program |  | 0.2 |
Regular benefits paid | (10.7) | (10.2) |
Benefit obligation, end of year | 158.4 | 147.9 | 158.8
Other Postretirement Plans | Spire Alabama |  |  |
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] |  |  |
Benefit obligation, beginning of year | 43.4 | 43.1 |
Service cost | 0.4 | 0.4 | 0.2
Interest cost | 1.4 | 1.8 | 1.5
Actuarial loss (gain) | (0.7) | 1.2 |
Plan amendments | 6.3 | 0.6 |
Regular benefits paid | (2.5) | (3.7) |
Benefit obligation, end of year | $ 48.3 | $ 43.4 | $ 43.1

X
- Definition Amount of actuarial present value of benefits attributed to employee service rendered, excluding assumptions about future compensation level. + 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 (e) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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- Definition Amount of gain (loss) from change in actuarial assumptions which (increases) decreases benefit obligation of defined benefit plan. Assumptions include, but are not limited to, interest, mortality, employee turnover, salary, and temporary deviation from substantive 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 (a)(4) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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- Definition Amount of actuarial present value of benefits attributed to service rendered by employee for 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 (a) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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- Definition Amount of payment to participant of defined benefit plan which decreases benefit obligation. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. + 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 (a)(6) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 60 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6414203&loc=d3e39716-114964 + Details
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- Definition Amount of (increase) decrease to benefit obligation of defined benefit plan from remeasurement due to settlement. Excludes decrease to benefit obligation for settlement payment. + 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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- 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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- Definition Amount, before benefit payment, of receipt from prescription drug subsidy which increases benefit obligation of defined benefit postretirement plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 60 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6414203&loc=d3e39716-114964 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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