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REGULATORY MATTERS - Schedule of Regulatory Assets and Liabilities (Details) - USD ($)
$ in Millions
Mar. 31, 2016
Sep. 30, 2015
Mar. 31, 2015
Regulatory Assets [Line Items]      
Regulatory Assets: Current $ 32.4 $ 27.6 $ 27.4
Regulatory Assets: Non-current 732.6 737.6 632.9
Total Regulatory Assets 773.9 778.1 660.3
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 33.8 32.4 38.5
Regulatory Liabilities: Non-current 110.7 119.3 114.3
Total Regulatory Liabilities 148.8 179.9 229.5
RSE adjustment      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 5.9 12.2 9.9
Unbilled service margin      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 14.9 6.4 15.0
Refundable negative salvage      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 9.2 10.8 10.8
Regulatory Liabilities: Non-current 10.1 16.2 17.0
Unamortized purchased gas adjustments      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 4.3 28.2 76.7
Other      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 3.8 3.0 2.8
Regulatory Liabilities: Non-current 14.5 15.5 12.3
Postretirement liabilities      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 27.8 28.9 25.1
Accrued cost of removal      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 58.3 58.7 59.9
Total Regulatory Liabilities (current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 38.1 60.6 115.2
Total Regulatory Liabilities (non-current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 110.7 119.3 114.3
Pension and postretirement benefit costs      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 26.9 22.0 21.4
Regulatory Assets: Non-current 441.0 448.7 420.8
Unamortized purchased gas adjustments      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 8.9 12.9 0.0
Other      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 5.5 5.6 6.0
Regulatory Assets: Non-current 30.3 29.1 28.4
Future income taxes due from customers      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 142.3 134.5 125.6
Accrued cost of removal      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 80.2 78.9 19.1
Purchased gas costs      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 15.1 24.1 18.6
Energy efficiency      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 23.7 22.3 20.4
Total Regulatory Assets (current)      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 41.3 40.5 27.4
Total Regulatory Assets (non-current)      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 732.6 737.6 632.9
Laclede Gas      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 23.7 16.2 17.4
Regulatory Assets: Non-current 563.8 573.6 545.3
Total Regulatory Assets 591.4 602.7 562.7
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 1.3 0.6 0.0
Regulatory Liabilities: Non-current 69.3 70.6 71.7
Total Regulatory Liabilities 74.9 71.2 111.4
Laclede Gas | Unamortized purchased gas adjustments      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 4.3 0.0 39.1
Laclede Gas | Other      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 1.3 0.6 0.6
Regulatory Liabilities: Non-current 11.0 11.9 11.8
Laclede Gas | Accrued cost of removal      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 58.3 58.7 59.9
Laclede Gas | Total Regulatory Liabilities (current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 5.6 0.6 39.7
Laclede Gas | Total Regulatory Liabilities (non-current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 69.3 70.6 71.7
Laclede Gas | Pension and postretirement benefit costs      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 20.2 15.5 15.0
Regulatory Assets: Non-current 357.1 368.0 357.9
Laclede Gas | Unamortized purchased gas adjustments      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 3.9 12.9 0.0
Laclede Gas | Other      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 3.5 0.7 2.4
Regulatory Assets: Non-current 25.6 24.7 22.8
Laclede Gas | Future income taxes due from customers      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 142.3 134.5 125.6
Laclede Gas | Purchased gas costs      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 15.1 24.1 18.6
Laclede Gas | Energy efficiency      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 23.7 22.3 20.4
Laclede Gas | Total Regulatory Assets (current)      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 27.6 29.1 17.4
Laclede Gas | Total Regulatory Assets (non-current)      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 563.8 573.6 545.3
Alagasco      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 8.7 11.4 10.0
Regulatory Assets: Non-current 168.1 163.6 87.6
Total Regulatory Assets 181.8 175.0 97.6
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 32.5 31.8 37.9
Regulatory Liabilities: Non-current 41.4 48.7 42.8
Total Regulatory Liabilities 73.9 108.7 118.3
Alagasco | RSE adjustment      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 5.9 12.2 9.9
Alagasco | Unbilled service margin      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 14.9 6.4 15.0
Alagasco | Refundable negative salvage      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 9.2 10.8 10.8
Regulatory Liabilities: Non-current 10.1 16.2 17.0
Alagasco | Unamortized purchased gas adjustments      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 0.0 28.2 37.6
Alagasco | Other      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 2.5 2.4 2.2
Regulatory Liabilities: Non-current 3.5 3.6 0.7
Alagasco | Postretirement liabilities      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 27.8 28.9 25.1
Alagasco | Total Regulatory Liabilities (current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 32.5 60.0 75.5
Alagasco | Total Regulatory Liabilities (non-current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 41.4 48.7 42.8
Alagasco | Pension and postretirement benefit costs      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 6.7 6.5 6.4
Regulatory Assets: Non-current 83.9 80.7 62.9
Alagasco | Unamortized purchased gas adjustments      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 5.0 0.0 0.0
Alagasco | Other      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 2.0 4.9 3.6
Regulatory Assets: Non-current 4.0 4.0 5.6
Alagasco | Accrued cost of removal      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 80.2 78.9 19.1
Alagasco | Total Regulatory Assets (current)      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 13.7 11.4 10.0
Alagasco | Total Regulatory Assets (non-current)      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current $ 168.1 $ 163.6 $ 87.6