FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) - USD ($) $ in Millions	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 13.8	$ 16.1		$ 53.0		$ 27.5
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	13.8	16.1
Short-term debt	338.0	287.1
Long-term debt, including current portion	1,851.5	1,851.0
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	13.8	16.1
Short-term debt	338.0	287.1
Long-term debt, including current portion	1,944.2	1,937.3
Laclede Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1.7	3.7		$ 23.9		$ 2.4
Laclede Gas | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1.7	3.7
Short-term debt	233.0	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	1.7	3.7
Short-term debt	233.0	238.6
Long-term debt, including current portion	880.2	876.2
Alagasco
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	7.2	5.6	$ 3.0		$ 5.6
Alagasco | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	7.2	5.6
Short-term debt	31.0	16.0
Long-term debt, including current portion	250.0	249.8
Alagasco | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	7.2	5.6
Short-term debt	31.0	16.0
Long-term debt, including current portion	263.2	266.4
Quoted Prices in Active Markets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	13.8	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	1.7	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	7.2	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	338.0	287.1
Long-term debt, including current portion	1,944.2	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	233.0	238.6
Long-term debt, including current portion	880.2	876.2
Significant Observable Inputs (Level 2) | Alagasco
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.0	0.0
Short-term debt	31.0	16.0
Long-term debt, including current portion	263.2	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