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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