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Pension Plans and Other Postretirement Benefits - Amounts Recognized in Balance Sheets (Details) - USD ($) $ in Millions | Sep. 30, 2020 | Sep. 30, 2019
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Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract] |  |
Noncurrent liabilities | $ (309.0) | $ (264.8)
Spire Missouri |  |
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract] |  |
Noncurrent liabilities | (217.2) | (192.4)
Spire Alabama |  |
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract] |  |
Noncurrent liabilities | (74.9) | (59.2)
Pension Plans |  |
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract] |  |
Current liabilities | (0.6) | (0.5)
Noncurrent liabilities | (258.9) | (229.1)
Total | (259.5) | (229.6)
Pension Plans | Spire Missouri |  |
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract] |  |
Current liabilities | (0.6) | (0.5)
Noncurrent liabilities | (168.4) | (158.7)
Total | (169.0) | (159.2)
Pension Plans | Spire Alabama |  |
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract] |  |
Current liabilities | 0.0 | 0.0
Noncurrent liabilities | (74.9) | (59.2)
Total | (74.9) | (59.2)
Other Postretirement Plans |  |
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract] |  |
Noncurrent assets | 129.0 | 118.3
Current liabilities | (0.5) | (0.5)
Noncurrent liabilities | (49.8) | (35.3)
Total | 78.7 | 82.5
Other Postretirement Plans | Spire Missouri |  |
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract] |  |
Noncurrent assets | 90.3 | 75.3
Current liabilities | (0.5) | (0.5)
Noncurrent liabilities | (49.0) | (33.8)
Total | 40.8 | 41.0
Other Postretirement Plans | Spire Alabama |  |
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract] |  |
Noncurrent assets | 38.7 | 43.0
Current liabilities | 0.0 | 0.0
Noncurrent liabilities | 0.0 | 0.0
Total | $ 38.7 | $ 43.0

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- Definition Amount of asset (liability), recognized in statement of financial position, for defined benefit pension and other postretirement plans. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=109237824&loc=d3e1703-114919 + Details
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- Definition Amount of asset, recognized in statement of financial position, for overfunded defined benefit pension and other postretirement plans. + 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 (c) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=109237824&loc=d3e1703-114919 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(17)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(10)) -URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910 Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=109237824&loc=d3e1731-114919 Reference 7: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(10)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 + Details
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- Definition Amount of liability, recognized in statement of financial position, for defined benefit pension and other postretirement plans, classified as current. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=109237824&loc=d3e1731-114919 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(20)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=109237824&loc=d3e1703-114919 + Details
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