REGULATORY MATTERS - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2015	Sep. 30, 2014
Regulatory Assets [Line Items]
Regulatory assets, current	$ 27.6	$ 26.8
Regulatory assets, non current	737.6	614.3
Total Regulatory Assets	778.1	695.1
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	32.4	41.3
Regulatory liabilities, noncurrent	119.3	125.8
Total Regulatory Liabilities	179.9	189.5
RSE adjustment
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	12.2	19.8
Unbilled service margin
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	6.4	5.2
Pension and postretirement costs
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	10.8	13.4
Regulatory liabilities, noncurrent	16.2	26.8
Unamortized purchased gas adjustments
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	28.2	22.4
Postretirement liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities, noncurrent	28.9	26.2
Accrued cost of removal
Regulatory Liabilities [Line Items]
Regulatory liabilities, noncurrent	58.7	60.5
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	3.0	2.9
Regulatory liabilities, noncurrent	15.5	12.3
Total Current Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	60.6	63.7
Total Noncurrent Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities, noncurrent	119.3	125.8
Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Regulatory assets, current	22.0	21.4
Regulatory assets, non current	448.7	431.5
Unamortized purchased gas adjustments
Regulatory Assets [Line Items]
Regulatory assets, current	12.9	54.0
Future income taxes due from customers
Regulatory Assets [Line Items]
Regulatory assets, non current	134.5	117.0
Accrued cost of removal
Regulatory Assets [Line Items]
Regulatory assets, non current	78.9	21.2
Purchased gas costs
Regulatory Assets [Line Items]
Regulatory assets, non current	24.1	4.3
Energy efficiency
Regulatory Assets [Line Items]
Regulatory assets, non current	22.3	18.9
Other
Regulatory Assets [Line Items]
Regulatory assets, current	5.6	5.4
Regulatory assets, non current	29.1	21.4
Total Current Regulatory Assets
Regulatory Assets [Line Items]
Regulatory assets, current	40.5	80.8
Total Noncurrent Regulatory Assets
Regulatory Assets [Line Items]
Regulatory assets, non current	737.6	614.3
Laclede Gas
Regulatory Assets [Line Items]
Regulatory assets, current	16.2	18.0
Regulatory assets, non current	573.6	523.7
Total Regulatory Assets	602.7	595.7
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	0.6	0.6
Regulatory liabilities, noncurrent	70.6	72.1
Total Regulatory Liabilities	71.2	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 | Pension and postretirement costs
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	0.0	0.0
Regulatory liabilities, noncurrent	0.0	0.0
Laclede Gas | Unamortized purchased gas adjustments
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	0.0	0.0
Laclede Gas | Postretirement liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities, noncurrent	0.0	0.0
Laclede Gas | Accrued cost of removal
Regulatory Liabilities [Line Items]
Regulatory liabilities, noncurrent	58.7	60.5
Laclede Gas | Other
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	0.6	0.6
Regulatory liabilities, noncurrent	11.9	11.6
Laclede Gas | Total Current Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	0.6	0.6
Laclede Gas | Total Noncurrent Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities, noncurrent	70.6	72.1
Laclede Gas | Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Regulatory assets, current	15.5	15.0
Regulatory assets, non current	368.0	365.4
Laclede Gas | Unamortized purchased gas adjustments
Regulatory Assets [Line Items]
Regulatory assets, current	12.9	54.0
Laclede Gas | Future income taxes due from customers
Regulatory Assets [Line Items]
Regulatory assets, non current	134.5	117.0
Laclede Gas | Accrued cost of removal
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	24.1	4.3
Laclede Gas | Energy efficiency
Regulatory Assets [Line Items]
Regulatory assets, non current	22.3	18.9
Laclede Gas | Other
Regulatory Assets [Line Items]
Regulatory assets, current	0.7	3.0
Regulatory assets, non current	24.7	18.1
Laclede Gas | Total Current Regulatory Assets
Regulatory Assets [Line Items]
Regulatory assets, current	29.1	72.0
Laclede Gas | Total Noncurrent Regulatory Assets
Regulatory Assets [Line Items]
Regulatory assets, non current	573.6	523.7
Alagasco
Regulatory Assets [Line Items]
Regulatory assets, current	11.4	8.8
Regulatory assets, non current	163.6	90.6
Total Regulatory Assets	175.0	99.4
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	31.8	40.7
Regulatory liabilities, noncurrent	48.7	53.7
Total Regulatory Liabilities	108.7	116.8
Alagasco | RSE adjustment
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	12.2	19.8
Alagasco | Unbilled service margin
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	6.4	5.2
Alagasco | Pension and postretirement costs
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	10.8	13.4
Regulatory liabilities, noncurrent	16.2	26.8
Alagasco | Unamortized purchased gas adjustments
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	28.2	22.4
Alagasco | Postretirement liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities, noncurrent	28.9	26.2
Alagasco | Accrued cost of removal
Regulatory Liabilities [Line Items]
Regulatory liabilities, noncurrent	0.0	0.0
Alagasco | Other
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	2.4	2.3
Regulatory liabilities, noncurrent	3.6	0.7
Alagasco | Total Current Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	60.0	63.1
Alagasco | Total Noncurrent Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities, noncurrent	48.7	53.7
Alagasco | Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Regulatory assets, current	6.5	6.4
Regulatory assets, non current	80.7	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 | Accrued cost of removal
Regulatory Assets [Line Items]
Regulatory assets, non current	78.9	21.2
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	4.9	2.4
Regulatory assets, non current	4.0	3.3
Alagasco | Total Current Regulatory Assets
Regulatory Assets [Line Items]
Regulatory assets, current	11.4	8.8
Alagasco | Total Noncurrent Regulatory Assets
Regulatory Assets [Line Items]
Regulatory assets, non current	$ 163.6	$ 90.6