FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) - USD ($) $ in Millions	Jun. 30, 2016	Sep. 30, 2015	Jun. 30, 2015	Sep. 30, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 4.9	$ 13.8	$ 5.7	$ 16.1
Laclede Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1.5	1.7	3.0	3.7
Alagasco
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.0	7.2	0.2	$ 5.6
Quoted Prices in Active Markets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	4.9	13.8	5.7
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	1.5	1.7	3.0
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	0.0	7.2	0.2
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	97.6	338.0	211.4
Long-term debt, including current portion	2,014.3	1,944.2	1,888.4
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	136.4	233.0	135.2
Long-term debt, including current portion	901.8	880.2	868.5
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	37.8	31.0	8.5
Long-term debt, including current portion	272.5	263.2	226.7
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
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	4.9	13.8	5.7
Short-term debt	97.6	338.0	211.4
Long-term debt, including current portion	1,851.7	1,851.5	1,816.4
Carrying Amount | Laclede Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1.5	1.7	3.0
Short-term debt	136.4	233.0	135.2
Long-term debt, including current portion	808.3	808.1	808.1
Carrying Amount | Alagasco
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.0	7.2	0.2
Short-term debt	37.8	31.0	8.5
Long-term debt, including current portion	250.0	250.0	215.0
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	4.9	13.8	5.7
Short-term debt	97.6	338.0	211.4
Long-term debt, including current portion	2,014.3	1,944.2	1,888.4
Fair Value | Laclede Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	1.5	1.7	3.0
Short-term debt	136.4	233.0	135.2
Long-term debt, including current portion	901.8	880.2	868.5
Fair Value | Alagasco
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.0	7.2	0.2
Short-term debt	37.8	31.0	8.5
Long-term debt, including current portion	$ 272.5	$ 263.2	$ 226.7