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REGULATORY MATTERS - Schedule of Regulatory Assets and Liabilities (Details) - USD ($)
$ in Millions
Jun. 30, 2017
Sep. 30, 2016
Jun. 30, 2016
Regulatory Assets [Line Items]      
Regulatory Assets: Current $ 70.7 $ 44.2 $ 37.5
Regulatory Assets: Non-current 840.5 838.0 730.7
Total Regulatory Assets 985.1 931.9 773.6
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 24.9 28.9 22.4
Regulatory Liabilities: Non-current 139.8 130.7 105.7
Total Regulatory Liabilities 165.7 161.3 128.1
Rate Stabilization and Equalization (RSE) adjustment      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 1.2 7.5 2.9
Unbilled service margin      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 6.0 5.9 6.4
Pension and postretirement benefit costs      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 27.0 28.9 27.2
Refundable negative salvage      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 8.2 9.3 9.3
Regulatory Liabilities: Non-current 4.1 9.4 9.3
Unamortized purchased gas adjustments      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 1.0 1.7 0.0
Other      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 9.5 6.2 3.8
Regulatory Liabilities: Non-current 34.1 17.6 13.8
Accrued cost of removal      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 74.6 74.8 55.4
Total Regulatory Liabilities (current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 25.9 30.6 22.4
Total Regulatory Liabilities (non-current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 139.8 130.7 105.7
Pension and postretirement benefit costs      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 41.7 27.0 26.9
Regulatory Assets: Non-current 453.4 487.9 434.4
Unamortized purchased gas adjustments      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 73.9 49.7 5.4
Regulatory Assets: Non-current 13.9 12.6 14.7
Other      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 29.0 17.2 10.6
Regulatory Assets: Non-current 43.4 30.1 29.6
Total Regulatory Assets (current)      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 144.6 93.9 42.9
Future income taxes due from customers      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 166.6 151.3 146.4
Accrued cost of removal      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 135.4 130.6 81.2
Energy efficiency      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 27.8 25.5 24.4
Total Regulatory Assets (non-current)      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 840.5 838.0 730.7
Laclede Gas      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 38.2 23.9 23.7
Regulatory Assets: Non-current 583.4 589.8 562.6
Total Regulatory Assets 650.6 656.8 586.9
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 2.7 1.3 1.3
Regulatory Liabilities: Non-current 83.6 67.3 65.7
Total Regulatory Liabilities 86.3 68.6 67.0
Laclede Gas | Other      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 2.7 1.3 1.3
Regulatory Liabilities: Non-current 28.8 12.2 10.3
Laclede Gas | Accrued cost of removal      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 54.8 55.1 55.4
Laclede Gas | Total Regulatory Liabilities (current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 2.7 1.3 1.3
Laclede Gas | Total Regulatory Liabilities (non-current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 83.6 67.3 65.7
Laclede Gas | Pension and postretirement benefit costs      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 34.9 20.2 20.2
Regulatory Assets: Non-current 352.3 375.7 352.1
Laclede Gas | Unamortized purchased gas adjustments      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 29.0 43.1 0.6
Regulatory Assets: Non-current 13.9 12.6 14.7
Laclede Gas | Other      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 3.3 3.7 3.5
Regulatory Assets: Non-current 22.8 24.7 25.0
Laclede Gas | Total Regulatory Assets (current)      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 67.2 67.0 24.3
Laclede Gas | Future income taxes due from customers      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 166.6 151.3 146.4
Laclede Gas | Energy efficiency      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 27.8 25.5 24.4
Laclede Gas | Total Regulatory Assets (non-current)      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 583.4 589.8 562.6
Alagasco      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 17.3 14.9 13.8
Regulatory Assets: Non-current 228.7 230.7 167.5
Total Regulatory Assets 290.9 251.2 186.1
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 17.8 22.7 21.1
Regulatory Liabilities: Non-current 34.4 41.7 40.0
Total Regulatory Liabilities 52.2 64.4 61.1
Alagasco | Rate Stabilization and Equalization (RSE) adjustment      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 1.2 5.0 2.9
Alagasco | Unbilled service margin      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 6.0 5.9 6.4
Alagasco | Pension and postretirement benefit costs      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 27.0 28.9 27.2
Alagasco | Refundable negative salvage      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 8.2 9.3 9.3
Regulatory Liabilities: Non-current 4.1 9.4 9.3
Alagasco | Other      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 2.4 2.5 2.5
Regulatory Liabilities: Non-current 3.3 3.4 3.5
Alagasco | Total Regulatory Liabilities (current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 17.8 22.7 21.1
Alagasco | Total Regulatory Liabilities (non-current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 34.4 41.7 40.0
Alagasco | Pension and postretirement benefit costs      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 6.8 6.8 6.7
Regulatory Assets: Non-current 92.2 98.9 82.3
Alagasco | Unamortized purchased gas adjustments      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 44.9 5.6 4.8
Alagasco | Other      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 10.5 8.1 7.1
Regulatory Assets: Non-current 1.1 1.2 4.0
Alagasco | Total Regulatory Assets (current)      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 62.2 20.5 18.6
Alagasco | Accrued cost of removal      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 135.4 130.6 81.2
Alagasco | Total Regulatory Assets (non-current)      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current $ 228.7 $ 230.7 $ 167.5