FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) - USD ($) $ in Millions	Jun. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 8.3	$ 5.2	$ 4.9	$ 13.8
Laclede Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2.6	2.1	1.5	1.7
Alagasco
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.1	0.0	0.0	$ 7.2
Quoted Prices in Active Markets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	8.3	5.2	4.9
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) | Laclede Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2.6	2.1	1.5
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
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	450.7	398.7	97.6
Long-term debt, including current portion	2,033.0	2,257.1	2,014.3
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	0.0
Short-term debt	260.2	243.7	136.4
Long-term debt, including current portion	883.9	900.4	901.8
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	114.9	82.0	37.8
Long-term debt, including current portion	267.6	275.5	272.5
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	8.3	5.2	4.9
Short-term debt	450.7	398.7	97.6
Long-term debt, including current portion	1,925.3	2,070.7	1,839.8
Carrying Amount | Laclede Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2.6	2.1	1.5
Short-term debt	260.2	243.7	136.4
Long-term debt, including current portion	804.5	804.1	803.9
Carrying Amount | Alagasco
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.1
Short-term debt	114.9	82.0	37.8
Long-term debt, including current portion	247.7	247.6	247.6
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	8.3	5.2	4.9
Short-term debt	450.7	398.7	97.6
Long-term debt, including current portion	2,033.0	2,257.1	2,014.3
Fair Value | Laclede Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2.6	2.1	1.5
Short-term debt	260.2	243.7	136.4
Long-term debt, including current portion	883.9	900.4	901.8
Fair Value | Alagasco
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.1
Short-term debt	114.9	82.0	37.8
Long-term debt, including current portion	$ 267.6	$ 275.5	$ 272.5