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REGULATORY MATTERS - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Millions | Sep. 30, 2017 | Sep. 30, 2016
--- | --- | ---
Regulatory Liabilities [Line Items] |  |
Total Current Regulatory Liabilities | $ 22.6 | $ 30.6
Total Noncurrent Regulatory Liabilities | 157.2 | 130.7
Total Regulatory Liabilities | 179.8 | 161.3
RSE adjustment |  |
Regulatory Liabilities [Line Items] |  |
Total Current Regulatory Liabilities | 1.4 | 7.5
Pension and postretirement costs |  |
Regulatory Liabilities [Line Items] |  |
Total Current Regulatory Liabilities | 8.2 | 9.3
Total Noncurrent Regulatory Liabilities | 4.1 | 9.4
Unbilled service margin |  |
Regulatory Liabilities [Line Items] |  |
Total Current Regulatory Liabilities | 0.0 | 5.9
Unamortized purchased gas adjustments |  |
Regulatory Liabilities [Line Items] |  |
Total Current Regulatory Liabilities | 1.0 | 1.7
Other |  |
Regulatory Liabilities [Line Items] |  |
Total Current Regulatory Liabilities | 12.0 | 6.2
Total Noncurrent Regulatory Liabilities | 37.1 | 17.6
Pension and postretirement benefit costs |  |
Regulatory Liabilities [Line Items] |  |
Total Noncurrent Regulatory Liabilities | 32.2 | 28.9
Accrued cost of removal |  |
Regulatory Liabilities [Line Items] |  |
Total Noncurrent Regulatory Liabilities | 83.8 | 74.8
Spire Missouri |  |
Regulatory Liabilities [Line Items] |  |
Total Current Regulatory Liabilities | 2.7 | 1.3
Total Noncurrent Regulatory Liabilities | 81.2 | 67.3
Total Regulatory Liabilities | 83.9 | 68.6
Spire Missouri | RSE adjustment |  |
Regulatory Liabilities [Line Items] |  |
Total Current Regulatory Liabilities | 0.0 | 0.0
Spire Missouri | Pension and postretirement costs |  |
Regulatory Liabilities [Line Items] |  |
Total Current Regulatory Liabilities | 0.0 | 0.0
Total Noncurrent Regulatory Liabilities | 0.0 | 0.0
Spire Missouri | Unbilled service margin |  |
Regulatory Liabilities [Line Items] |  |
Total Current Regulatory Liabilities | 0.0 | 0.0
Spire Missouri | Unamortized purchased gas adjustments |  |
Regulatory Liabilities [Line Items] |  |
Total Current Regulatory Liabilities | 0.0 | 0.0
Spire Missouri | Other |  |
Regulatory Liabilities [Line Items] |  |
Total Current Regulatory Liabilities | 2.7 | 1.3
Total Noncurrent Regulatory Liabilities | 26.7 | 12.2
Spire Missouri | Pension and postretirement benefit costs |  |
Regulatory Liabilities [Line Items] |  |
Total Noncurrent Regulatory Liabilities | 0.0 | 0.0
Spire Missouri | Accrued cost of removal |  |
Regulatory Liabilities [Line Items] |  |
Total Noncurrent Regulatory Liabilities | 54.5 | 55.1
Spire Alabama |  |
Regulatory Liabilities [Line Items] |  |
Total Current Regulatory Liabilities | 12.0 | 22.7
Total Noncurrent Regulatory Liabilities | 39.6 | 41.7
Total Regulatory Liabilities | 51.6 | 64.4
Spire Alabama | RSE adjustment |  |
Regulatory Liabilities [Line Items] |  |
Total Current Regulatory Liabilities | 1.4 | 5.0
Spire Alabama | Pension and postretirement costs |  |
Regulatory Liabilities [Line Items] |  |
Total Current Regulatory Liabilities | 8.2 | 9.3
Total Noncurrent Regulatory Liabilities | 4.1 | 9.4
Spire Alabama | Unbilled service margin |  |
Regulatory Liabilities [Line Items] |  |
Total Current Regulatory Liabilities | 0.0 | 5.9
Spire Alabama | Unamortized purchased gas adjustments |  |
Regulatory Liabilities [Line Items] |  |
Total Current Regulatory Liabilities | 0.0 | 0.0
Spire Alabama | Other |  |
Regulatory Liabilities [Line Items] |  |
Total Current Regulatory Liabilities | 2.4 | 2.5
Total Noncurrent Regulatory Liabilities | 3.3 | 3.4
Spire Alabama | Pension and postretirement benefit costs |  |
Regulatory Liabilities [Line Items] |  |
Total Noncurrent Regulatory Liabilities | 32.2 | 28.9
Spire Alabama | Accrued cost of removal |  |
Regulatory Liabilities [Line Items] |  |
Total Noncurrent Regulatory Liabilities | $ 0.0 | $ 0.0

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- Definition The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 405 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394 + Details
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- Definition The amount for the individual regulatory current liability as itemized in a table of regulatory current liabilities as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 405 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394 + Details
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