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Regulatory Matters - Schedule of Regulatory Assets Not Earning a Return (Details) - USD ($) $ in Millions | Sep. 30, 2020 | Sep. 30, 2019
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Regulatory Assets [Line Items] |  |
Total Regulatory Assets Not Earning a Return | $ 366.5 | $ 334.2
Spire Missouri |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets Not Earning a Return | 359.8 | 328.3
Pension and Other Postretirement Benefits |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets Not Earning a Return | 232.3 | 211.1
Pension and Other Postretirement Benefits | Spire Missouri |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets Not Earning a Return | 232.3 | 211.1
Future Income Taxes Due from Customers |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets Not Earning a Return | 121.3 | 108.8
Future Income Taxes Due from Customers | Spire Missouri |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets Not Earning a Return | 114.6 | 102.9
Other |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets Not Earning a Return | 12.9 | 14.3
Other | Spire Missouri |  |
Regulatory Assets [Line Items] |  |
Total Regulatory Assets Not Earning a Return | $ 12.9 | $ 14.3

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- Definition Remaining balance of regulatory assets for which no return on investment during recovery is provided. In some cases, a regulator may permit an enterprise to include a cost that would be charged to expense by an unregulated enterprise as an allowable cost over a period of time by amortizing that cost for rate-making purposes, but the regulator does not include the unrecovered amount in the rate base. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 55 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=121594574&loc=d3e44360-110383 + Details
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