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REGULATORY MATTERS - Schedule of Regulatory Assets and Liabilities (Details) - USD ($)
$ in Millions
Mar. 31, 2017
Sep. 30, 2016
Mar. 31, 2016
Regulatory Assets [Line Items]      
Regulatory Assets: Current $ 64.8 $ 44.2 $ 32.4
Regulatory Assets: Non-current 827.7 838.0 732.6
Total Regulatory Assets 953.5 931.9 773.9
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 29.7 28.9 33.8
Regulatory Liabilities: Non-current 144.1 130.7 110.7
Total Regulatory Liabilities 175.4 161.3 148.8
Rate Stabilization and Equalization (RSE) adjustment      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 1.1 7.5 5.9
Unbilled service margin      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 13.5 5.9 14.9
Pension and postretirement benefit costs      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 27.6 28.9 27.8
Refundable negative salvage      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 8.1 9.3 9.2
Regulatory Liabilities: Non-current 4.9 9.4 10.1
Unamortized purchased gas adjustments      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 1.6 1.7 4.3
Other      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 7.0 6.2 3.8
Regulatory Liabilities: Non-current 36.3 17.6 14.5
Accrued cost of removal      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 75.3 74.8 58.3
Total Regulatory Liabilities (current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 31.3 30.6 38.1
Total Regulatory Liabilities (non-current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 144.1 130.7 110.7
Pension and postretirement benefit costs      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 41.7 27.0 26.9
Regulatory Assets: Non-current 461.8 487.9 441.0
Unamortized purchased gas adjustments      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 61.0 49.7 8.9
Regulatory Assets: Non-current 1.1 12.6 15.1
Other      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 23.1 17.2 5.5
Regulatory Assets: Non-current 44.2 30.1 30.3
Future income taxes due from customers      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 159.7 151.3 142.3
Accrued cost of removal      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 133.9 130.6 80.2
Energy efficiency      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 27.0 25.5 23.7
Total Regulatory Assets (current)      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 125.8 93.9 41.3
Total Regulatory Assets (non-current)      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 827.7 838.0 732.6
Laclede Gas      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 38.2 23.9 23.7
Regulatory Assets: Non-current 569.4 589.8 563.8
Total Regulatory Assets 624.7 656.8 591.4
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 2.7 1.3 1.3
Regulatory Liabilities: Non-current 84.5 67.3 69.3
Total Regulatory Liabilities 87.2 68.6 74.9
Laclede Gas | Unamortized purchased gas adjustments      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 0.0 0.0 4.3
Laclede Gas | Other      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 2.7 1.3 1.3
Regulatory Liabilities: Non-current 28.9 12.2 11.0
Laclede Gas | Accrued cost of removal      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 55.6 55.1 58.3
Laclede Gas | Total Regulatory Liabilities (current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 2.7 1.3 5.6
Laclede Gas | Total Regulatory Liabilities (non-current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 84.5 67.3 69.3
Laclede Gas | Pension and postretirement benefit costs      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 34.8 20.2 20.2
Regulatory Assets: Non-current 358.3 375.7 357.1
Laclede Gas | Unamortized purchased gas adjustments      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 17.1 43.1 3.9
Regulatory Assets: Non-current 1.1 12.6 15.1
Laclede Gas | Other      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 3.4 3.7 3.5
Regulatory Assets: Non-current 23.3 24.7 25.6
Laclede Gas | Future income taxes due from customers      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 159.7 151.3 142.3
Laclede Gas | Energy efficiency      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 27.0 25.5 23.7
Laclede Gas | Total Regulatory Assets (current)      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 55.3 67.0 27.6
Laclede Gas | Total Regulatory Assets (non-current)      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 569.4 589.8 563.8
Alagasco      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 11.2 14.9 8.7
Regulatory Assets: Non-current 229.3 230.7 168.1
Total Regulatory Assets 284.4 251.2 181.8
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 25.1 22.7 32.5
Regulatory Liabilities: Non-current 35.9 41.7 41.4
Total Regulatory Liabilities 61.0 64.4 73.9
Alagasco | Rate Stabilization and Equalization (RSE) adjustment      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 1.1 5.0 5.9
Alagasco | Unbilled service margin      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 13.5 5.9 14.9
Alagasco | Pension and postretirement benefit costs      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 27.6 28.9 27.8
Alagasco | Refundable negative salvage      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 8.1 9.3 9.2
Regulatory Liabilities: Non-current 4.9 9.4 10.1
Alagasco | Other      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 2.4 2.5 2.5
Regulatory Liabilities: Non-current 3.4 3.4 3.5
Alagasco | Total Regulatory Liabilities (current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 25.1 22.7 32.5
Alagasco | Total Regulatory Liabilities (non-current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 35.9 41.7 41.4
Alagasco | Pension and postretirement benefit costs      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 6.8 6.8 6.7
Regulatory Assets: Non-current 94.4 98.9 83.9
Alagasco | Unamortized purchased gas adjustments      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 43.9 5.6 5.0
Alagasco | Other      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 4.4 8.1 2.0
Regulatory Assets: Non-current 1.0 1.2 4.0
Alagasco | Accrued cost of removal      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 133.9 130.6 80.2
Alagasco | Total Regulatory Assets (current)      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 55.1 20.5 13.7
Alagasco | Total Regulatory Assets (non-current)      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current $ 229.3 $ 230.7 $ 168.1