FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 13.4	$ 16.1	$ 34.5	$ 53.0
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	13.4	16.1
Short-term debt	397.5	287.1
Long-term debt, including current portion	1,851.0	1,851.0
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	13.4	16.1
Short-term debt	397.5	287.1
Long-term debt, including current portion	1,975.0	1,937.3
Laclede Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	5.9	3.7	7.9	23.9
Laclede Gas | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	5.9	3.7
Short-term debt	319.5	238.6
Long-term debt, including current portion	808.0	807.9
Laclede Gas | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	5.9	3.7
Short-term debt	319.5	238.6
Long-term debt, including current portion	892.2	876.2
Alagasco
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3.3	5.6	3.0	8.5
Alagasco | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3.3	5.6
Short-term debt	21.5	16.0
Long-term debt, including current portion	249.7	249.8
Alagasco | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3.3	5.6
Short-term debt	21.5	16.0
Long-term debt, including current portion	268.8	266.4
Quoted Prices in Active Markets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	13.4	16.1
Short-term debt	0	0
Long-term debt, including current portion	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	5.9	3.7
Short-term debt	0	0
Long-term debt, including current portion	0	0
Quoted Prices in Active Markets (Level 1) | Alagasco
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3.3	5.6
Short-term debt	0	0
Long-term debt, including current portion	0	0
Significant Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
Short-term debt	397.5	287.1
Long-term debt, including current portion	1,975.0	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
Short-term debt	319.5	238.6
Long-term debt, including current portion	892.2	876.2
Significant Observable Inputs (Level 2) | Alagasco
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
Short-term debt	21.5	16.0
Long-term debt, including current portion	268.8	266.4
Significant Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
Short-term debt	0	0
Long-term debt, including current portion	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
Short-term debt	0	0
Long-term debt, including current portion	0	0
Significant Unobservable Inputs (Level 3) | Alagasco
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
Short-term debt	0	0
Long-term debt, including current portion	$ 0	$ 0