NOTES PAYABLE AND CREDIT AGREEMENTS - Laclede Gas' Short-term Borrowings (Details) - USD ($)	12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Short-term Debt [Line Items]
Weighted average borrowings outstanding	$ 300,600,000	$ 82,300,000
Weighted average interest rate	0.70%	0.50%
Minimum borrowing outstanding	$ 180,100,000.0	$ 0
Maximum borrowing outstanding	488,500,000.0	300,500,000.0
Short-term debt	$ 338,000,000	$ 287,100,000
Weighted average interest rate	0.80%	0.50%
Laclede Gas
Short-term Debt [Line Items]
Weighted average borrowings outstanding	$ 213,000,000	$ 141,000,000
Weighted average interest rate	0.40%	0.30%
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	$ 233,000,000	$ 238,600,000
Weighted average interest rate	0.50%	0.30%
Laclede Gas | Commercial Paper
Short-term Debt [Line Items]
Weighted average borrowings outstanding	$ 212,700,000	$ 77,600,000
Weighted average interest rate	0.40%	0.30%
Minimum borrowing outstanding	$ 102,100,000.0	$ 0
Maximum borrowing outstanding	341,000,000.0	244,500,000.0
Short-term debt	$ 233,000,000	$ 238,600,000
Weighted average interest rate	0.50%	0.30%
Laclede Gas | Notes Payable to Related Party
Short-term Debt [Line Items]
Weighted average borrowings outstanding	$ 300,000	$ 63,400,000
Weighted average interest rate	0.50%	0.30%
Minimum borrowing outstanding	$ 0	$ 0
Maximum borrowing outstanding	10,400,000.0	189,000,000.0
Short-term debt	$ 0	$ 0
Weighted average interest rate	0.00%	0.00%