FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) - USD ($) $ in Millions	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 4.6	$ 13.8	$ 13.4	$ 16.1
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	4.6	13.8	13.4
Short-term debt	377.1	338.0	397.5
Long-term debt, including current portion	1,851.5	1,851.5	1,851.0
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	4.6	13.8	13.4
Short-term debt	377.1	338.0	397.5
Long-term debt, including current portion	1,916.5	1,944.2	1,975.0
Laclede Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1.3	1.7	5.9	3.7
Laclede Gas | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1.3	1.7	5.9
Short-term debt	274.1	233.0	319.5
Long-term debt, including current portion	808.2	808.1	808.0
Laclede Gas | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1.3	1.7	5.9
Short-term debt	274.1	233.0	319.5
Long-term debt, including current portion	858.5	880.2	892.2
Alagasco
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.1	7.2	3.3	$ 5.6
Alagasco | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.1	7.2	3.3
Short-term debt	43.0	31.0	21.5
Long-term debt, including current portion	250.0	250.0	249.7
Alagasco | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.1	7.2	3.3
Short-term debt	43.0	31.0	21.5
Long-term debt, including current portion	256.5	263.2	268.8
Quoted Prices in Active Markets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	4.6	13.8	13.4
Short-term debt	0.0	0.0	0.0
Long-term debt, including current portion	0.0	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	1.3	1.7	5.9
Short-term debt	0.0	0.0	0.0
Long-term debt, including current portion	0.0	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	0.1	7.2	3.3
Short-term debt	0.0	0.0	0.0
Long-term debt, including current portion	0.0	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	0.0
Short-term debt	377.1	338.0	397.5
Long-term debt, including current portion	1,916.5	1,944.2	1,975.0
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	0.0
Short-term debt	274.1	233.0	319.5
Long-term debt, including current portion	858.5	880.2	892.2
Significant Observable Inputs (Level 2) | Alagasco
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.0	0.0	0.0
Short-term debt	43.0	31.0	21.5
Long-term debt, including current portion	256.5	263.2	268.8
Significant Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.0	0.0	0.0
Short-term debt	0.0	0.0	0.0
Long-term debt, including current portion	0.0	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	0.0
Short-term debt	0.0	0.0	0.0
Long-term debt, including current portion	0.0	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	0.0
Short-term debt	0.0	0.0	0.0
Long-term debt, including current portion	$ 0.0	$ 0.0	$ 0.0