FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) - USD ($) $ in Millions	Mar. 31, 2017	Sep. 30, 2016	Mar. 31, 2016	Sep. 30, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 19.6	$ 5.2	$ 8.7	$ 13.8
Laclede Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3.7	2.1	3.8	1.7
Alagasco
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0.0	0.0	2.3	$ 7.2
Quoted Prices in Active Markets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	19.6	5.2	8.7
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	3.7	2.1	3.8
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			2.3
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	567.4	398.7	253.6
Long-term debt, including current portion	1,993.9	2,257.1	1,960.4
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	282.2	243.7	169.6
Long-term debt, including current portion	870.1	900.4	885.9
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	109.3	82.0	41.0
Long-term debt, including current portion	261.6	275.5	262.7
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	19.6	5.2	8.7
Short-term debt	567.4	398.7	253.6
Long-term debt, including current portion	1,925.3	1,820.7	1,839.3
Carrying Amount | Laclede Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3.7	2.1	3.8
Short-term debt	282.2	243.7	169.6
Long-term debt, including current portion	804.3	804.1	803.7
Carrying Amount | Alagasco
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents			2.3
Short-term debt	109.3	82.0	41.0
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	19.6	5.2	8.7
Short-term debt	567.4	398.7	253.6
Long-term debt, including current portion	1,993.9	2,257.1	1,960.4
Fair Value | Laclede Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3.7	2.1	3.8
Short-term debt	282.2	243.7	169.6
Long-term debt, including current portion	870.1	900.4	885.9
Fair Value | Alagasco
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents			2.3
Short-term debt	109.3	82.0	41.0
Long-term debt, including current portion	$ 261.6	$ 275.5	$ 262.7