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Regulatory Matters - Schedule of Regulatory Assets Not Earning a Return (Details) - USD ($) $ in Millions | Mar. 31, 2021 | Sep. 30, 2020 | Mar. 31, 2020
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Regulatory Assets [Line Items] |  |  |
Total Regulatory Assets Not Earning a Return | $ 474.3 | $ 366.5 | $ 353.4
Pension and Other Postretirement Benefits |  |  |
Regulatory Assets [Line Items] |  |  |
Total Regulatory Assets Not Earning a Return | 226.3 | 232.3 | 226.2
Future Income Taxes Due from Customers |  |  |
Regulatory Assets [Line Items] |  |  |
Total Regulatory Assets Not Earning a Return | 125.1 | 121.3 | 113.5
Other |  |  |
Regulatory Assets [Line Items] |  |  |
Total Regulatory Assets Not Earning a Return | 122.9 | 12.9 | 13.7
Spire Missouri |  |  |
Regulatory Assets [Line Items] |  |  |
Total Regulatory Assets Not Earning a Return | 467.8 | 359.8 | 346.7
Spire Missouri | Pension and Other Postretirement Benefits |  |  |
Regulatory Assets [Line Items] |  |  |
Total Regulatory Assets Not Earning a Return | 226.3 | 232.3 | 226.2
Spire Missouri | Future Income Taxes Due from Customers |  |  |
Regulatory Assets [Line Items] |  |  |
Total Regulatory Assets Not Earning a Return | 118.6 | 114.6 | 106.8
Spire Missouri | Other |  |  |
Regulatory Assets [Line Items] |  |  |
Total Regulatory Assets Not Earning a Return | $ 122.9 | $ 12.9 | $ 13.7

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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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