REGULATORY MATTERS - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2016	Sep. 30, 2015
Regulatory Assets [Line Items]
Regulatory assets, current	$ 93.9	$ 40.5
Regulatory assets, non current	838.0	737.6
Total Regulatory Assets	931.9	778.1
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	30.6	60.6
Regulatory liabilities, noncurrent	130.7	119.3
Total Regulatory Liabilities	161.3	179.9
RSE adjustment
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	7.5	12.2
Unbilled service margin
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	5.9	6.4
Pension and postretirement costs
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	9.3	10.8
Regulatory liabilities, noncurrent	9.4	16.2
Unamortized purchased gas adjustments
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	1.7	28.2
Postretirement liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities, noncurrent	28.9	28.9
Accrued cost of removal
Regulatory Liabilities [Line Items]
Regulatory liabilities, noncurrent	74.8	58.7
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	6.2	3.0
Regulatory liabilities, noncurrent	17.6	15.5
Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Regulatory assets, current	27.0	22.0
Regulatory assets, non current	487.9	448.7
Unamortized purchased gas adjustments
Regulatory Assets [Line Items]
Regulatory assets, current	49.7	12.9
Future income taxes due from customers
Regulatory Assets [Line Items]
Regulatory assets, non current	151.3	134.5
Accrued cost of removal
Regulatory Assets [Line Items]
Regulatory assets, non current	130.6	78.9
Purchased gas costs
Regulatory Assets [Line Items]
Regulatory assets, non current	12.6	24.1
Energy efficiency
Regulatory Assets [Line Items]
Regulatory assets, non current	25.5	22.3
Other
Regulatory Assets [Line Items]
Regulatory assets, current	17.2	5.6
Regulatory assets, non current	30.1	29.1
Laclede Gas
Regulatory Assets [Line Items]
Regulatory assets, current	67.0	29.1
Regulatory assets, non current	589.8	573.6
Total Regulatory Assets	656.8	602.7
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	1.3	0.6
Regulatory liabilities, noncurrent	67.3	70.6
Total Regulatory Liabilities	68.6	71.2
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	55.1	58.7
Laclede Gas | Other
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	1.3	0.6
Regulatory liabilities, noncurrent	12.2	11.9
Laclede Gas | Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Regulatory assets, current	20.2	15.5
Regulatory assets, non current	375.7	368.0
Laclede Gas | Unamortized purchased gas adjustments
Regulatory Assets [Line Items]
Regulatory assets, current	43.1	12.9
Laclede Gas | Future income taxes due from customers
Regulatory Assets [Line Items]
Regulatory assets, non current	151.3	134.5
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	12.6	24.1
Laclede Gas | Energy efficiency
Regulatory Assets [Line Items]
Regulatory assets, non current	25.5	22.3
Laclede Gas | Other
Regulatory Assets [Line Items]
Regulatory assets, current	3.7	0.7
Regulatory assets, non current	24.7	24.7
Alagasco
Regulatory Assets [Line Items]
Regulatory assets, current	20.5	11.4
Regulatory assets, non current	230.7	163.6
Total Regulatory Assets	251.2	175.0
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	22.7	60.0
Regulatory liabilities, noncurrent	41.7	48.7
Total Regulatory Liabilities	64.4	108.7
Alagasco | RSE adjustment
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	5.0	12.2
Alagasco | Unbilled service margin
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	5.9	6.4
Alagasco | Pension and postretirement costs
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	9.3	10.8
Regulatory liabilities, noncurrent	9.4	16.2
Alagasco | Unamortized purchased gas adjustments
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	0.0	28.2
Alagasco | Postretirement liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities, noncurrent	28.9	28.9
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	2.5	2.4
Regulatory liabilities, noncurrent	3.4	3.6
Alagasco | Pension and postretirement benefit costs
Regulatory Assets [Line Items]
Regulatory assets, current	6.8	6.5
Regulatory assets, non current	98.9	80.7
Alagasco | Unamortized purchased gas adjustments
Regulatory Assets [Line Items]
Regulatory assets, current	5.6	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	130.6	78.9
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	8.1	4.9
Regulatory assets, non current	$ 1.2	$ 4.0