REGULATORY MATTERS - Schedule of Regulatory Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Sep. 30, 2014
Regulatory Assets [Line Items]
Regulatory Assets: Current	$ 27.4	$ 26.8
Regulatory Assets: Non-current	632.9	614.3
Total Regulatory Assets	660.3	695.1
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	38.5	41.3
Regulatory Liabilities: Non-current	114.3	125.8
Total Regulatory Liabilities	229.5	189.5
RSE adjustment
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	9.9	19.8
Unbilled service margin
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	15.0	5.2
Refundable negative salvage
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	10.8	13.4
Regulatory Liabilities: Non-current	17.0	26.8
Unamortized purchased gas adjustments
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	76.7	22.4
Other
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	2.8	2.9
Regulatory Liabilities: Non-current	12.3	12.3
Postretirement liabilities
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Non-current	25.1	26.2
Accrued cost of removal
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Non-current	59.9	60.5
Total Regulatory Liabilities (current)
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	115.2	63.7
Total Regulatory Liabilities (non-current)
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Non-current	114.3	125.8
Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Regulatory Assets: Current	21.4	21.4
Regulatory Assets: Non-current	420.8	431.5
Unamortized purchased gas adjustments
Regulatory Assets [Line Items]
Regulatory Assets: Current	0	54.0
Future income taxes due from customers
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	125.6	117.0
Accretion and depreciation of asset retirement obligations
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	19.1	18.4
Purchased gas costs
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	18.6	4.3
Energy efficiency
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	20.4	18.9
Other
Regulatory Assets [Line Items]
Regulatory Assets: Current	6.0	5.4
Regulatory Assets: Non-current	28.4	24.2
Total Regulatory Assets (current)
Regulatory Assets [Line Items]
Regulatory Assets: Current	27.4	80.8
Total Regulatory Assets (non-current)
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	632.9	614.3
Laclede Gas
Regulatory Assets [Line Items]
Regulatory Assets: Current	17.4	18.0
Regulatory Assets: Non-current	545.3	523.7
Total Regulatory Assets	562.7	595.7
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Non-current	71.7	72.1
Total Regulatory Liabilities	111.4	72.7
Laclede Gas | RSE adjustment
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	0	0
Laclede Gas | Unbilled service margin
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	0	0
Laclede Gas | Refundable negative salvage
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	0	0
Regulatory Liabilities: Non-current	0	0
Laclede Gas | Unamortized purchased gas adjustments
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	39.1	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
Laclede Gas | Accrued cost of removal
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Non-current	59.9	60.5
Laclede Gas | Total Regulatory Liabilities (current)
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	39.7	0.6
Laclede Gas | Total Regulatory Liabilities (non-current)
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Non-current	71.7	72.1
Laclede Gas | Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Regulatory Assets: Current	15.0	15.0
Regulatory Assets: Non-current	357.9	365.4
Laclede Gas | Unamortized purchased gas adjustments
Regulatory Assets [Line Items]
Regulatory Assets: Current	0	54.0
Laclede Gas | Future income taxes due from customers
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	125.6	117.0
Laclede Gas | Accretion and depreciation of asset retirement obligations
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	0	0
Laclede Gas | Purchased gas costs
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	18.6	4.3
Laclede Gas | Energy efficiency
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	20.4	18.9
Laclede Gas | Other
Regulatory Assets [Line Items]
Regulatory Assets: Current	2.4	3.0
Regulatory Assets: Non-current	22.8	18.1
Laclede Gas | Total Regulatory Assets (current)
Regulatory Assets [Line Items]
Regulatory Assets: Current	17.4	72.0
Laclede Gas | Total Regulatory Assets (non-current)
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	545.3	523.7
Alagasco
Regulatory Assets [Line Items]
Regulatory Assets: Current	10.0	8.8
Regulatory Assets: Non-current	87.6	90.6
Total Regulatory Assets	97.6	99.4
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	37.9	40.7
Regulatory Liabilities: Non-current	42.8	53.7
Total Regulatory Liabilities	118.3	116.8
Alagasco | RSE adjustment
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	9.9	19.8
Alagasco | Unbilled service margin
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	15.0	5.2
Alagasco | Refundable negative salvage
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	10.8	13.4
Regulatory Liabilities: Non-current	17.0	26.8
Alagasco | Unamortized purchased gas adjustments
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	37.6	22.4
Alagasco | Other
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	2.2	2.3
Regulatory Liabilities: Non-current	0.7	0.7
Alagasco | Postretirement liabilities
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Non-current	25.1	26.2
Alagasco | Accrued cost of removal
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Non-current	0	0
Alagasco | Total Regulatory Liabilities (current)
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	75.5	63.1
Alagasco | Total Regulatory Liabilities (non-current)
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Non-current	42.8	53.7
Alagasco | Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Regulatory Assets: Current	6.4	6.4
Regulatory Assets: Non-current	62.9	66.1
Alagasco | Unamortized purchased gas adjustments
Regulatory Assets [Line Items]
Regulatory Assets: Current	0	0
Alagasco | Future income taxes due from customers
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	0	0
Alagasco | Accretion and depreciation of asset retirement obligations
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	19.1	18.4
Alagasco | Purchased gas costs
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	0	0
Alagasco | Energy efficiency
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	0	0
Alagasco | Other
Regulatory Assets [Line Items]
Regulatory Assets: Current	3.6	2.4
Regulatory Assets: Non-current	5.6	6.1
Alagasco | Total Regulatory Assets (current)
Regulatory Assets [Line Items]
Regulatory Assets: Current	10.0	8.8
Alagasco | Total Regulatory Assets (non-current)
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	$ 87.6	$ 90.6