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REGULATORY MATTERS - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions | Dec. 31, 2016 | Sep. 30, 2016 | Dec. 31, 2015
--- | --- | --- | ---
Regulatory Assets [Line Items] |  |  |
Regulatory Assets: Current | $ 82.3 | $ 44.2 | $ 31.7
Regulatory Assets: Non-current | 786.4 | 838.0 | 727.0
Total Regulatory Assets | 920.9 | 931.9 | 803.3
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities: Current | 42.8 | 28.9 | 41.5
Regulatory Liabilities: Non-current | 132.1 | 130.7 | 129.1
Total Regulatory Liabilities | 176.3 | 161.3 | 184.9
Rate Stabilization and Equalization (RSE) adjustment |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities: Current | 3.8 | 7.5 | 11.1
Unbilled service margin |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities: Current | 22.0 | 5.9 | 16.4
Refundable negative salvage |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities: Current | 9.0 | 9.3 | 10.5
Regulatory Liabilities: Non-current | 8.9 | 9.4 | 15.8
Unamortized purchased gas adjustments |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities: Current | 1.4 | 1.7 | 14.3
Other |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities: Current | 8.0 | 6.2 | 3.5
Regulatory Liabilities: Non-current | 20.2 | 17.6 | 26.3
Postretirement liabilities |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities: Non-current | 28.3 | 28.9 | 28.4
Accrued cost of removal |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities: Non-current | 74.7 | 74.8 | 58.6
Total Regulatory Liabilities (current) |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities: Current | 44.2 | 30.6 | 55.8
Total Regulatory Liabilities (non-current) |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities: Non-current | 132.1 | 130.7 | 129.1
Pension and postretirement benefit costs |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets: Current | 63.2 | 27.0 | 26.7
Regulatory Assets: Non-current | 439.2 | 487.9 | 441.3
Unamortized purchased gas adjustments |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets: Current | 52.2 | 49.7 | 44.6
Other |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets: Current | 19.1 | 17.2 | 5.0
Regulatory Assets: Non-current | 29.4 | 30.1 | 33.1
Future income taxes due from customers |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets: Non-current | 155.5 | 151.3 | 138.7
Accrued cost of removal |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets: Non-current | 131.6 | 130.6 | 79.4
Purchased gas costs |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets: Non-current | 4.7 | 12.6 | 11.5
Energy efficiency |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets: Non-current | 26.0 | 25.5 | 23.0
Total Regulatory Assets (current) |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets: Current | 134.5 | 93.9 | 76.3
Total Regulatory Assets (non-current) |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets: Non-current | 786.4 | 838.0 | 727.0
Laclede Gas |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets: Current | 59.7 | 23.9 | 20.9
Regulatory Assets: Non-current | 543.4 | 589.8 | 563.9
Total Regulatory Assets | 636.9 | 656.8 | 629.4
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities: Current | 2.7 | 1.3 | 1.1
Regulatory Liabilities: Non-current | 67.3 | 67.3 | 81.4
Total Regulatory Liabilities | 70.0 | 68.6 | 82.5
Laclede Gas | Other |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities: Current | 2.7 | 1.3 | 1.1
Regulatory Liabilities: Non-current | 12.5 | 12.2 | 22.8
Laclede Gas | Accrued cost of removal |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities: Non-current | 54.8 | 55.1 | 58.6
Laclede Gas | Total Regulatory Liabilities (current) |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities: Current | 2.7 | 1.3 | 1.1
Laclede Gas | Total Regulatory Liabilities (non-current) |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities: Non-current | 67.3 | 67.3 | 81.4
Laclede Gas | Pension and postretirement benefit costs |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets: Current | 56.3 | 20.2 | 20.2
Regulatory Assets: Non-current | 333.3 | 375.7 | 362.2
Laclede Gas | Unamortized purchased gas adjustments |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets: Current | 33.8 | 43.1 | 44.6
Laclede Gas | Other |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets: Current | 3.4 | 3.7 | 0.7
Regulatory Assets: Non-current | 23.9 | 24.7 | 28.5
Laclede Gas | Future income taxes due from customers |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets: Non-current | 155.5 | 151.3 | 138.7
Laclede Gas | Purchased gas costs |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets: Non-current | 4.7 | 12.6 | 11.5
Laclede Gas | Energy efficiency |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets: Non-current | 26.0 | 25.5 | 23.0
Laclede Gas | Total Regulatory Assets (current) |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets: Current | 93.5 | 67.0 | 65.5
Laclede Gas | Total Regulatory Assets (non-current) |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets: Non-current | 543.4 | 589.8 | 563.9
Alagasco |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets: Current | 14.4 | 14.9 | 10.8
Regulatory Assets: Non-current | 229.5 | 230.7 | 162.5
Total Regulatory Assets | 261.0 | 251.2 | 173.3
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities: Current | 37.4 | 22.7 | 40.4
Regulatory Liabilities: Non-current | 40.6 | 41.7 | 47.7
Total Regulatory Liabilities | 78.0 | 64.4 | 102.4
Alagasco | Rate Stabilization and Equalization (RSE) adjustment |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities: Current | 3.8 | 5.0 | 11.1
Alagasco | Unbilled service margin |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities: Current | 22.0 | 5.9 | 16.4
Alagasco | Refundable negative salvage |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities: Current | 9.0 | 9.3 | 10.5
Regulatory Liabilities: Non-current | 8.9 | 9.4 | 15.8
Alagasco | Unamortized purchased gas adjustments |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities: Current | 0.0 | 0.0 | 14.3
Alagasco | Other |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities: Current | 2.6 | 2.5 | 2.4
Regulatory Liabilities: Non-current | 3.4 | 3.4 | 3.5
Alagasco | Postretirement liabilities |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities: Non-current | 28.3 | 28.9 | 28.4
Alagasco | Total Regulatory Liabilities (current) |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities: Current | 37.4 | 22.7 | 54.7
Alagasco | Total Regulatory Liabilities (non-current) |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities: Non-current | 40.6 | 41.7 | 47.7
Alagasco | Pension and postretirement benefit costs |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets: Current | 6.8 | 6.8 | 6.5
Regulatory Assets: Non-current | 96.8 | 98.9 | 79.1
Alagasco | Unamortized purchased gas adjustments |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets: Current | 17.1 | 5.6 | 0.0
Alagasco | Other |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets: Current | 7.6 | 8.1 | 4.3
Regulatory Assets: Non-current | 1.1 | 1.2 | 4.0
Alagasco | Accrued cost of removal |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets: Non-current | 131.6 | 130.6 | 79.4
Alagasco | Total Regulatory Assets (current) |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets: Current | 31.5 | 20.5 | 10.8
Alagasco | Total Regulatory Assets (non-current) |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets: Non-current | $ 229.5 | $ 230.7 | $ 162.5

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