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REGULATORY MATTERS - Schedule of Regulatory Assets and Liabilities (Details) - USD ($)
$ in Millions
Jun. 30, 2015
Sep. 30, 2014
Regulatory Assets [Line Items]    
Regulatory Assets: Current $ 27.0 $ 26.8
Regulatory Assets: Non-current 644.6 614.3
Total Regulatory Assets 671.6 695.1
Regulatory Liabilities [Line Items]    
Regulatory Liabilities: Current 29.4 41.3
Regulatory Liabilities: Non-current 114.9 125.8
Total Regulatory Liabilities 196.6 189.5
RSE adjustment    
Regulatory Liabilities [Line Items]    
Regulatory Liabilities: Current 10.3 19.8
Unbilled service margin    
Regulatory Liabilities [Line Items]    
Regulatory Liabilities: Current 5.4 5.2
Refundable negative salvage    
Regulatory Liabilities [Line Items]    
Regulatory Liabilities: Current 10.8 13.4
Regulatory Liabilities: Non-current 16.2 26.8
Unamortized purchased gas adjustments    
Regulatory Liabilities [Line Items]    
Regulatory Liabilities: Current 52.3 22.4
Other    
Regulatory Liabilities [Line Items]    
Regulatory Liabilities: Current 2.9 2.9
Regulatory Liabilities: Non-current 15.1 12.3
Postretirement liabilities    
Regulatory Liabilities [Line Items]    
Regulatory Liabilities: Non-current 24.5 26.2
Accrued cost of removal    
Regulatory Liabilities [Line Items]    
Regulatory Liabilities: Non-current 59.1 60.5
Total Regulatory Liabilities (current)    
Regulatory Liabilities [Line Items]    
Regulatory Liabilities: Current 81.7 63.7
Total Regulatory Liabilities (non-current)    
Regulatory Liabilities [Line Items]    
Regulatory Liabilities: Non-current 114.9 125.8
Pension and postretirement benefit costs    
Regulatory Assets [Line Items]    
Regulatory Assets: Current 21.6 21.4
Regulatory Assets: Non-current 424.6 431.5
Unamortized purchased gas adjustments    
Regulatory Assets [Line Items]    
Regulatory Assets: Current 0.0 54.0
Future income taxes due from customers    
Regulatory Assets [Line Items]    
Regulatory Assets: Non-current 130.3 117.0
Accretion and depreciation of asset retirement obligations    
Regulatory Assets [Line Items]    
Regulatory Assets: Non-current 19.5 18.4
Purchased gas costs    
Regulatory Assets [Line Items]    
Regulatory Assets: Non-current 20.9 4.3
Energy efficiency    
Regulatory Assets [Line Items]    
Regulatory Assets: Non-current 21.3 18.9
Other    
Regulatory Assets [Line Items]    
Regulatory Assets: Current 5.4 5.4
Regulatory Assets: Non-current 28.0 24.2
Total Regulatory Assets (current)    
Regulatory Assets [Line Items]    
Regulatory Assets: Current 27.0 80.8
Total Regulatory Assets (non-current)    
Regulatory Assets [Line Items]    
Regulatory Assets: Non-current 644.6 614.3
Laclede Gas    
Regulatory Assets [Line Items]    
Regulatory Assets: Current 17.5 18.0
Regulatory Assets: Non-current 558.0 523.7
Total Regulatory Assets 575.5 595.7
Regulatory Liabilities [Line Items]    
Regulatory Liabilities: Non-current 70.9 72.1
Total Regulatory Liabilities 92.3 72.7
Laclede Gas | RSE adjustment    
Regulatory Liabilities [Line Items]    
Regulatory Liabilities: Current 0.0 0.0
Laclede Gas | Unbilled service margin    
Regulatory Liabilities [Line Items]    
Regulatory Liabilities: Current 0.0 0.0
Laclede Gas | Refundable negative salvage    
Regulatory Liabilities [Line Items]    
Regulatory Liabilities: Current 0.0 0.0
Regulatory Liabilities: Non-current 0.0 0.0
Laclede Gas | Unamortized purchased gas adjustments    
Regulatory Liabilities [Line Items]    
Regulatory Liabilities: Current 20.8 0.0
Laclede Gas | Other    
Regulatory Liabilities [Line Items]    
Regulatory Liabilities: Current 0.6 0.6
Regulatory Liabilities: Non-current 11.8 11.6
Laclede Gas | Postretirement liabilities    
Regulatory Liabilities [Line Items]    
Regulatory Liabilities: Non-current 0.0 0.0
Laclede Gas | Accrued cost of removal    
Regulatory Liabilities [Line Items]    
Regulatory Liabilities: Non-current 59.1 60.5
Laclede Gas | Total Regulatory Liabilities (current)    
Regulatory Liabilities [Line Items]    
Regulatory Liabilities: Current 21.4 0.6
Laclede Gas | Total Regulatory Liabilities (non-current)    
Regulatory Liabilities [Line Items]    
Regulatory Liabilities: Non-current 70.9 72.1
Laclede Gas | Pension and postretirement benefit costs    
Regulatory Assets [Line Items]    
Regulatory Assets: Current 15.2 15.0
Regulatory Assets: Non-current 362.9 365.4
Laclede Gas | Unamortized purchased gas adjustments    
Regulatory Assets [Line Items]    
Regulatory Assets: Current 0.0 54.0
Laclede Gas | Future income taxes due from customers    
Regulatory Assets [Line Items]    
Regulatory Assets: Non-current 130.3 117.0
Laclede Gas | Accretion and depreciation of asset retirement obligations    
Regulatory Assets [Line Items]    
Regulatory Assets: Non-current 0.0 0.0
Laclede Gas | Purchased gas costs    
Regulatory Assets [Line Items]    
Regulatory Assets: Non-current 20.9 4.3
Laclede Gas | Energy efficiency    
Regulatory Assets [Line Items]    
Regulatory Assets: Non-current 21.3 18.9
Laclede Gas | Other    
Regulatory Assets [Line Items]    
Regulatory Assets: Current 2.3 3.0
Regulatory Assets: Non-current 22.6 18.1
Laclede Gas | Total Regulatory Assets (current)    
Regulatory Assets [Line Items]    
Regulatory Assets: Current 17.5 72.0
Laclede Gas | Total Regulatory Assets (non-current)    
Regulatory Assets [Line Items]    
Regulatory Assets: Non-current 558.0 523.7
Alagasco    
Regulatory Assets [Line Items]    
Regulatory Assets: Current 9.5 8.8
Regulatory Assets: Non-current 86.6 90.6
Total Regulatory Assets 96.1 99.4
Regulatory Liabilities [Line Items]    
Regulatory Liabilities: Current 28.8 40.7
Regulatory Liabilities: Non-current 44.3 53.7
Total Regulatory Liabilities 104.6 116.8
Alagasco | RSE adjustment    
Regulatory Liabilities [Line Items]    
Regulatory Liabilities: Current 10.3 19.8
Alagasco | Unbilled service margin    
Regulatory Liabilities [Line Items]    
Regulatory Liabilities: Current 5.4 5.2
Alagasco | Refundable negative salvage    
Regulatory Liabilities [Line Items]    
Regulatory Liabilities: Current 10.8 13.4
Regulatory Liabilities: Non-current 16.2 26.8
Alagasco | Unamortized purchased gas adjustments    
Regulatory Liabilities [Line Items]    
Regulatory Liabilities: Current 31.5 22.4
Alagasco | Other    
Regulatory Liabilities [Line Items]    
Regulatory Liabilities: Current 2.3 2.3
Regulatory Liabilities: Non-current 3.6 0.7
Alagasco | Postretirement liabilities    
Regulatory Liabilities [Line Items]    
Regulatory Liabilities: Non-current 24.5 26.2
Alagasco | Accrued cost of removal    
Regulatory Liabilities [Line Items]    
Regulatory Liabilities: Non-current 0.0 0.0
Alagasco | Total Regulatory Liabilities (current)    
Regulatory Liabilities [Line Items]    
Regulatory Liabilities: Current 60.3 63.1
Alagasco | Total Regulatory Liabilities (non-current)    
Regulatory Liabilities [Line Items]    
Regulatory Liabilities: Non-current 44.3 53.7
Alagasco | Pension and postretirement benefit costs    
Regulatory Assets [Line Items]    
Regulatory Assets: Current 6.4 6.4
Regulatory Assets: Non-current 61.7 66.1
Alagasco | Unamortized purchased gas adjustments    
Regulatory Assets [Line Items]    
Regulatory Assets: Current 0.0 0.0
Alagasco | Future income taxes due from customers    
Regulatory Assets [Line Items]    
Regulatory Assets: Non-current 0.0 0.0
Alagasco | Accretion and depreciation of asset retirement obligations    
Regulatory Assets [Line Items]    
Regulatory Assets: Non-current 19.5 18.4
Alagasco | Purchased gas costs    
Regulatory Assets [Line Items]    
Regulatory Assets: Non-current 0.0 0.0
Alagasco | Energy efficiency    
Regulatory Assets [Line Items]    
Regulatory Assets: Non-current 0.0 0.0
Alagasco | Other    
Regulatory Assets [Line Items]    
Regulatory Assets: Current 3.1 2.4
Regulatory Assets: Non-current 5.4 6.1
Alagasco | Total Regulatory Assets (current)    
Regulatory Assets [Line Items]    
Regulatory Assets: Current 9.5 8.8
Alagasco | Total Regulatory Assets (non-current)    
Regulatory Assets [Line Items]    
Regulatory Assets: Non-current $ 86.6 $ 90.6