NOTES PAYABLE AND CREDIT AGREEMENTS - Narrative (Details)		1 Months Ended	12 Months Ended
	Sep. 02, 2014 USD ($) bank	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) bank	Sep. 30, 2014 USD ($)
Line of Credit Facility [Line Items]
Minimum borrowing outstanding			$ 180,100,000.0	$ 0
Maximum borrowing outstanding			488,500,000.0	300,500,000.0
Short-term debt		$ 287,100,000	$ 338,000,000	287,100,000
Speculative interest rate increase			1.00%
Decrease in cash flow with an impact of 100 basis point interest rate increase			$ 3,000,000
Laclede Gas
Line of Credit Facility [Line Items]
Minimum borrowing outstanding			104,200,000.0	45,500,000.0
Maximum borrowing outstanding			341,000,000.0	272,100,000.0
Short-term debt		238,600,000	$ 233,000,000	238,600,000
Speculative interest rate increase			1.00%
Decrease in cash flow with an impact of 100 basis point interest rate increase			$ 2,100,000
Alagasco
Line of Credit Facility [Line Items]
Speculative interest rate increase			1.00%
Decrease in cash flow with an impact of 100 basis point interest rate increase			$ 200,000
Line of Credit
Line of Credit Facility [Line Items]
Line of credit facility borrowing capacity			$ 150,000,000
Banks in syndicated line of credit facility | bank			9
Minimum required ratio of earnings before interest, taxes, depreciation, and amortization (EBITDA) times interest expense as required by the line of credit covenant			70.00%
Debt to total capitalization ratio			0.58
Minimum borrowing outstanding			$ 32,500,000.0	0
Maximum borrowing outstanding			80,000,000.0	40,000,000
Short-term debt		32,500,000	74,000,000	32,500,000
Line of Credit | Laclede Gas
Line of Credit Facility [Line Items]
Line of credit facility borrowing capacity			$ 450,000,000.0
Banks in syndicated line of credit facility | bank			9
Minimum required ratio of earnings before interest, taxes, depreciation, and amortization (EBITDA) times interest expense as required by the line of credit covenant			70.00%
Debt to total capitalization ratio			0.50
Minimum borrowing outstanding			$ 102,100,000.0	0
Maximum borrowing outstanding			341,000,000.0	244,500,000.0
Short-term debt		238,600,000	$ 233,000,000	238,600,000
Line of Credit | Alagasco
Line of Credit Facility [Line Items]
Line of credit facility borrowing capacity	$ 150,000,000.0
Banks in syndicated line of credit facility | bank	12
Minimum required ratio of earnings before interest, taxes, depreciation, and amortization (EBITDA) times interest expense as required by the line of credit covenant	70.00%
Debt to total capitalization ratio			0.24
Minimum borrowing outstanding		9,000,000.0	$ 0	0
Maximum borrowing outstanding		16,000,000.0	69,500,000.0	55,000,000.0
Short-term debt		$ 16,000,000	$ 31,000,000	$ 16,000,000
Line of Credit | Liabilities | Lender Concentration Risk
Line of Credit Facility [Line Items]
Percent of debt by a singe bank			15.60%
Line of Credit | Liabilities | Lender Concentration Risk | Laclede Gas
Line of Credit Facility [Line Items]
Percent of debt by a singe bank			15.60%
Line of Credit | Liabilities | Lender Concentration Risk | Alagasco
Line of Credit Facility [Line Items]
Percent of debt by a singe bank	10.00%