FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) - USD ($) $ in Millions	Jun. 30, 2015	Sep. 30, 2014	Jun. 30, 2014	Sep. 30, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 5.7	$ 16.1	$ 571.8	$ 53.0
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	5.7	16.1
Short-term debt	211.4	287.1
Long-term debt, including current portion	1,816.4	1,851.0
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	5.7	16.1
Short-term debt	211.4	287.1
Long-term debt, including current portion	1,888.4	1,937.3
Laclede Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3.0	3.7	6.0	23.9
Laclede Gas | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3.0	3.7
Short-term debt	135.2	238.6
Long-term debt, including current portion	808.1	807.9
Laclede Gas | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3.0	3.7
Short-term debt	135.2	238.6
Long-term debt, including current portion	868.5	876.2
Alagasco
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.2	5.6	$ 11.8	$ 8.5
Alagasco | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.2	5.6
Short-term debt	8.5	16.0
Long-term debt, including current portion	215.0	249.8
Alagasco | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.2	5.6
Short-term debt	8.5	16.0
Long-term debt, including current portion	226.7	266.4
Quoted Prices in Active Markets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	5.7	16.1
Short-term debt	0.0	0.0
Long-term debt, including current portion	0.0	0.0
Quoted Prices in Active Markets (Level 1) | Laclede Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3.0	3.7
Short-term debt	0.0	0.0
Long-term debt, including current portion	0.0	0.0
Quoted Prices in Active Markets (Level 1) | Alagasco
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.2	5.6
Short-term debt	0.0	0.0
Long-term debt, including current portion	0.0	0.0
Significant Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.0	0.0
Short-term debt	211.4	287.1
Long-term debt, including current portion	1,888.4	1,937.3
Significant Observable Inputs (Level 2) | Laclede Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.0	0.0
Short-term debt	135.2	238.6
Long-term debt, including current portion	868.5	876.2
Significant Observable Inputs (Level 2) | Alagasco
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.0	0.0
Short-term debt	8.5	16.0
Long-term debt, including current portion	226.7	266.4
Significant Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.0	0.0
Short-term debt	0.0	0.0
Long-term debt, including current portion	0.0	0.0
Significant Unobservable Inputs (Level 3) | Laclede Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.0	0.0
Short-term debt	0.0	0.0
Long-term debt, including current portion	0.0	0.0
Significant Unobservable Inputs (Level 3) | Alagasco
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.0	0.0
Short-term debt	0.0	0.0
Long-term debt, including current portion	$ 0.0	$ 0.0