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REGULATORY MATTERS - Schedule of Regulatory Assets Not Earnings a Return (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Sep. 30, 2017 | Dec. 31, 2016
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Regulatory Assets [Line Items] |  |  |
Total Regulatory Assets Not Earning a Return | $ 318.1 | $ 380.3 | $ 399.1
Future income taxes due from customers |  |  |
Regulatory Assets [Line Items] |  |  |
Total Regulatory Assets Not Earning a Return | 113.1 | 170.5 | 155.5
Pension and postretirement benefit costs |  |  |
Regulatory Assets [Line Items] |  |  |
Total Regulatory Assets Not Earning a Return | 193.8 | 198.5 | 231.4
Other |  |  |
Regulatory Assets [Line Items] |  |  |
Total Regulatory Assets Not Earning a Return | 11.2 | 11.3 | 12.2
Spire Missouri |  |  |
Regulatory Assets [Line Items] |  |  |
Total Regulatory Assets Not Earning a Return | 318.1 | 380.3 | 399.1
Spire Missouri | Future income taxes due from customers |  |  |
Regulatory Assets [Line Items] |  |  |
Total Regulatory Assets Not Earning a Return | 113.1 | 170.5 | 155.5
Spire Missouri | Pension and postretirement benefit costs |  |  |
Regulatory Assets [Line Items] |  |  |
Total Regulatory Assets Not Earning a Return | 193.8 | 198.5 | 231.4
Spire Missouri | Other |  |  |
Regulatory Assets [Line Items] |  |  |
Total Regulatory Assets Not Earning a Return | $ 11.2 | $ 11.3 | $ 12.2

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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://www.xbrl.org/2003/role/presentationRef -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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 55 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=82849221&loc=d3e44360-110383 + Details
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