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REGULATORY MATTERS - Schedule of Regulatory Assets and Liabilities (Details) - USD ($)
$ in Millions
Dec. 31, 2015
Sep. 30, 2015
Dec. 31, 2014
Regulatory Assets [Line Items]      
Regulatory Assets: Current $ 31.7 $ 27.6 $ 26.6
Regulatory Assets: Non-current 727.0 737.6 615.8
Total Regulatory Assets 803.3 778.1 703.0
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 41.5 32.4 53.8
Regulatory Liabilities: Non-current 129.1 119.3 122.9
Total Regulatory Liabilities 184.9 179.9 198.8
RSE adjustment      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 11.1 12.2 17.0
Unbilled service margin      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 16.4 6.4 20.8
Pension and postretirement benefit costs      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 0.0 0.0 2.3
Refundable negative salvage      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 10.5 10.8 13.1
Regulatory Liabilities: Non-current 15.8 16.2 25.6
Unamortized purchased gas adjustments      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 14.3 28.2 22.1
Other      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 3.5 3.0 0.6
Regulatory Liabilities: Non-current 26.3 15.5 12.4
Postretirement liabilities      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 28.4 28.9 25.6
Accrued cost of removal      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 58.6 58.7 59.3
Total Regulatory Liabilities (current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 55.8 60.6 75.9
Total Regulatory Liabilities (non-current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 129.1 119.3 122.9
Pension and postretirement benefit costs      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 26.7 22.0 21.4
Regulatory Assets: Non-current 441.3 448.7 426.1
Unamortized purchased gas adjustments      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 44.6 12.9 60.6
Other      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 5.0 5.6 5.2
Regulatory Assets: Non-current 33.1 29.1 39.8
Future income taxes due from customers      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 138.7 134.5 121.3
Accrued cost of removal      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 79.4 78.9 21.1
Enhanced stability reserve      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 0.0 0.0 3.3
Purchased gas costs      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 11.5 24.1 4.2
Energy efficiency      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 23.0 22.3 0.0
Total Regulatory Assets (current)      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 76.3 40.5 87.2
Total Regulatory Assets (non-current)      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 727.0 737.6 615.8
Laclede Gas      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 563.9 573.6 526.9
Total Regulatory Assets 629.4 602.7 605.5
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 1.1 0.6 0.6
Regulatory Liabilities: Non-current 81.4 70.6 70.9
Total Regulatory Liabilities 82.5 71.2 71.5
Laclede Gas | Other      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 1.1 0.6 0.6
Regulatory Liabilities: Non-current 22.8 11.9 11.6
Laclede Gas | Accrued cost of removal      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 58.6 58.7 59.3
Laclede Gas | Total Regulatory Liabilities (current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 1.1 0.6 0.6
Laclede Gas | Total Regulatory Liabilities (non-current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 81.4 70.6 70.9
Laclede Gas | Pension and postretirement benefit costs      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 20.2 15.5 15.0
Regulatory Assets: Non-current 362.2 368.0 361.6
Laclede Gas | Unamortized purchased gas adjustments      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 44.6 12.9 60.6
Laclede Gas | Other      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 0.7 0.7 3.0
Regulatory Assets: Non-current 28.5 24.7 39.8
Laclede Gas | Future income taxes due from customers      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 138.7 134.5 121.3
Laclede Gas | Purchased gas costs      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 11.5 24.1 4.2
Laclede Gas | Energy efficiency      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 23.0 22.3 0.0
Laclede Gas | Total Regulatory Assets (current)      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 65.5 29.1 78.6
Laclede Gas | Total Regulatory Assets (non-current)      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 563.9 573.6 526.9
Alagasco      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 10.8 11.4 8.6
Regulatory Assets: Non-current 162.5 163.6 88.9
Total Regulatory Assets 173.3 175.0 97.5
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 40.4 31.8 53.2
Regulatory Liabilities: Non-current 47.7 48.7 51.9
Total Regulatory Liabilities 102.4 108.7 127.2
Alagasco | RSE adjustment      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 11.1 12.2 17.0
Alagasco | Unbilled service margin      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 16.4 6.4 20.8
Alagasco | Pension and postretirement benefit costs      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 0.0 0.0 2.3
Alagasco | Refundable negative salvage      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 10.5 10.8 13.1
Regulatory Liabilities: Non-current 15.8 16.2 25.6
Alagasco | Unamortized purchased gas adjustments      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 14.3 28.2 22.1
Alagasco | Other      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 2.4 2.4 0.0
Regulatory Liabilities: Non-current 3.5 3.6 0.7
Alagasco | Postretirement liabilities      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 28.4 28.9 25.6
Alagasco | Total Regulatory Liabilities (current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Current 54.7 60.0 75.3
Alagasco | Total Regulatory Liabilities (non-current)      
Regulatory Liabilities [Line Items]      
Regulatory Liabilities: Non-current 47.7 48.7 51.9
Alagasco | Pension and postretirement benefit costs      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 6.5 6.5 6.4
Regulatory Assets: Non-current 79.1 80.7 64.5
Alagasco | Other      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 4.3 4.9 2.2
Regulatory Assets: Non-current 4.0 4.0 0.0
Alagasco | Accrued cost of removal      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 79.4 78.9 21.1
Alagasco | Enhanced stability reserve      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current 0.0 0.0 3.3
Alagasco | Total Regulatory Assets (current)      
Regulatory Assets [Line Items]      
Regulatory Assets: Current 10.8 11.4 8.6
Alagasco | Total Regulatory Assets (non-current)      
Regulatory Assets [Line Items]      
Regulatory Assets: Non-current $ 162.5 $ 163.6 $ 88.9