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Pension Plans and Other Postretirement Benefits - Assumed Medical Cost Trend Rates and Effect of 1% Changed (Details) - Other Postretirement Plans - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2020 | Sep. 30, 2019
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Rate to which the medical cost trend rate is assumed to decline (the ultimate medical cost trend rate) | 5.00% | 5.00%
Year the rate reaches the ultimate trend | 2025 | 2025
Assumed 1% change in the assumed medical cost trend rate [Abstract] |  |
Effect of 1% increase on net periodic postretirement benefit cost | $ 0.8 |
Effect of 1% increase on accumulated postretirement benefit obligation | 9.7 |
Effect of 1% decrease on net periodic postretirement benefit cost | (0.7) |
Effect of 1% decrease on accumulated postretirement benefit obligation | $ (8.7) |
Spire Missouri |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Medical cost trend assumed for next year | 6.50% | 6.75%
Assumed 1% change in the assumed medical cost trend rate [Abstract] |  |
Effect of 1% increase on net periodic postretirement benefit cost | $ 0.6 |
Effect of 1% increase on accumulated postretirement benefit obligation | 6.5 |
Effect of 1% decrease on net periodic postretirement benefit cost | (0.6) |
Effect of 1% decrease on accumulated postretirement benefit obligation | $ (5.9) |
Spire Alabama |  |
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Medical cost trend assumed for next year | 6.50% | 6.75%
Assumed 1% change in the assumed medical cost trend rate [Abstract] |  |
Effect of 1% increase on net periodic postretirement benefit cost | $ 0.2 |
Effect of 1% increase on accumulated postretirement benefit obligation | 2.6 |
Effect of 1% decrease on net periodic postretirement benefit cost | (0.1) |
Effect of 1% decrease on accumulated postretirement benefit obligation | $ (2.3) |

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- Definition Amount of decrease in accumulated postretirement benefit obligation from one-percentage-point decrease in assumed health care cost trend rate. + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (m) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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- Definition Amount of decrease in service and interest cost components of net periodic postretirement benefit cost from one-percentage-point decrease in assumed health care cost trend rate. + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (m) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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- Definition Amount of increase in accumulated postretirement benefit obligation from one-percentage-point increase in assumed health care cost trend rate. + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (m) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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- Definition Amount of increase in service and interest cost components of net periodic postretirement benefit cost from one-percentage-point increase in assumed health care cost trend rate. + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (m) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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- Definition Assumed rate, for next fiscal year, based on annual change in cost of health care cost benefits used to measure expected cost of benefits covered by defined benefit postretirement plan. Factors include, but are not limited to, estimate of health care inflation, change in health care utilization or delivery pattern, technological advances, and change in health status of participant. Excludes factors for change in composition of plan population by age and dependency status. + 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 (l) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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- Definition Ultimate trend rate for health care cost for defined benefit postretirement 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 (l) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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- Definition Year ultimate health care cost trend rate is expected to be reached, in CCYY format. + 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 (l) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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