REGULATORY MATTERS - Schedule of Regulatory Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014
Regulatory Assets [Line Items]
Regulatory Assets: Current	$ 87.2	$ 80.8
Regulatory Assets: Non-current	615.8	614.3
Total Regulatory Assets	703.0	695.1
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	75.9	63.7
Regulatory Liabilities: Non-current	122.9	125.8
Total Regulatory Liabilities	198.8	189.5
RSE adjustment
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	17.0	19.8
Unbilled service margin
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	20.8	5.2
Pension and postretirement benefit costs
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	2.3	2.3
Refundable negative salvage
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	13.1	13.4
Regulatory Liabilities: Non-current	25.6	26.8
Gas Supply Adjustment
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	22.1	22.4
Other
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	0.6	0.6
Unamortized investment tax credits
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Non-current	2.6	2.7
Postretirement liabilities
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Non-current	25.6	26.2
Rate recovery of asset removal cost
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Non-current	59.3	60.5
Other
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Non-current	9.8	9.6
Total Regulatory Liabilities (non-current)
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Non-current	122.9	125.8
Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Regulatory Assets: Current	21.4	21.4
Regulatory Assets: Non-current	426.1	431.5
Unamortized purchased gas adjustments
Regulatory Assets [Line Items]
Regulatory Assets: Current	60.6	54.0
Future income taxes due from customers
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	121.3	117.0
Rate recovery of asset removal cost
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	2.3	2.8
Accretion and depreciation of asset retirement obligations
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	18.8	18.4
Enhanced stability reserve
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	3.3	3.3
Purchased gas costs
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	4.2	4.3
Compensated absences
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	8.2	8.2
Other
Regulatory Assets [Line Items]
Regulatory Assets: Current	5.2	5.4
Regulatory Assets: Non-current	31.6	28.8
Total Regulatory Assets (non-current)
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	615.8	614.3
Laclede Gas
Regulatory Assets [Line Items]
Regulatory Assets: Current	78.6	72.0
Regulatory Assets: Non-current	526.9	523.7
Total Regulatory Assets	605.5	595.7
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	0.6	0.6
Regulatory Liabilities: Non-current	70.9	72.1
Total Regulatory Liabilities	71.5	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 | Pension and postretirement benefit costs
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 | Gas Supply Adjustment
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	0	0
Laclede Gas | Other
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	0.6	0.6
Laclede Gas | Unamortized investment tax credits
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Non-current	2.6	2.7
Laclede Gas | Postretirement liabilities
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Non-current	0	0
Laclede Gas | Rate recovery of asset removal cost
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Non-current	59.3	60.5
Laclede Gas | Other
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Non-current	9.0	8.9
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	15.0	15.0
Regulatory Assets: Non-current	361.6	365.4
Laclede Gas | Unamortized purchased gas adjustments
Regulatory Assets [Line Items]
Regulatory Assets: Current	60.6	54.0
Laclede Gas | Future income taxes due from customers
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	121.3	117.0
Laclede Gas | Rate recovery of asset removal cost
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	0	0
Laclede Gas | Accretion and depreciation of asset retirement obligations
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	0	0
Laclede Gas | Enhanced stability reserve
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	0	0
Laclede Gas | Purchased gas costs
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	4.2	4.3
Laclede Gas | Compensated absences
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	8.2	8.2
Laclede Gas | Other
Regulatory Assets [Line Items]
Regulatory Assets: Current	3.0	3.0
Regulatory Assets: Non-current	31.6	28.8
Laclede Gas | Total Regulatory Assets (non-current)
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	526.9	523.7
Alagasco
Regulatory Assets [Line Items]
Regulatory Assets: Current	8.6	8.8
Regulatory Assets: Non-current	88.9	90.6
Total Regulatory Assets	97.5	99.4
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	75.3	63.1
Regulatory Liabilities: Non-current	51.9	53.7
Total Regulatory Liabilities	127.2	116.8
Alagasco | RSE adjustment
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	17.0	19.8
Alagasco | Unbilled service margin
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	20.8	5.2
Alagasco | Pension and postretirement benefit costs
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	2.3	2.3
Alagasco | Refundable negative salvage
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	13.1	13.4
Regulatory Liabilities: Non-current	25.6	26.8
Alagasco | Gas Supply Adjustment
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	22.1	22.4
Alagasco | Other
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Current	0	0
Alagasco | Unamortized investment tax credits
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Non-current	0	0
Alagasco | Postretirement liabilities
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Non-current	25.6	26.2
Alagasco | Rate recovery of asset removal cost
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Non-current	0	0
Alagasco | Other
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Non-current	0.7	0.7
Alagasco | Total Regulatory Liabilities (non-current)
Regulatory Liabilities [Line Items]
Regulatory Liabilities: Non-current	51.9	53.7
Alagasco | Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Regulatory Assets: Current	6.4	6.4
Regulatory Assets: Non-current	64.5	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 | Rate recovery of asset removal cost
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	2.3	2.8
Alagasco | Accretion and depreciation of asset retirement obligations
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	18.8	18.4
Alagasco | Enhanced stability reserve
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	3.3	3.3
Alagasco | Purchased gas costs
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	0	0
Alagasco | Compensated absences
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	0	0
Alagasco | Other
Regulatory Assets [Line Items]
Regulatory Assets: Current	2.2	2.4
Regulatory Assets: Non-current	0	0
Alagasco | Total Regulatory Assets (non-current)
Regulatory Assets [Line Items]
Regulatory Assets: Non-current	$ 88.9	$ 90.6