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Summary of Significant Accounting Policies - Accrued Capital Expenditures (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018
Schedule Of Accrued Capital Expenditures Excluded From Statement Of Cash Flow [Line Items] |  |  |
Accruals for capital expenditures | $ 67.6 | $ 80.6 | $ 62.1
Spire Missouri |  |  |
Schedule Of Accrued Capital Expenditures Excluded From Statement Of Cash Flow [Line Items] |  |  |
Accruals for capital expenditures | 34.3 | 40.1 | 36.7
Spire Alabama |  |  |
Schedule Of Accrued Capital Expenditures Excluded From Statement Of Cash Flow [Line Items] |  |  |
Accruals for capital expenditures | $ 17.0 | $ 11.9 | $ 8.9

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- Definition Future cash outflow to pay for purchases of fixed assets that have occurred. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=121583591&loc=d3e4332-108586 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=121583591&loc=d3e4313-108586 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=121583591&loc=d3e4304-108586 + Details
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