NOTES PAYABLE AND CREDIT AGREEMENTS - Laclede Gas' Short-term Borrowings (Details) - USD ($)	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Short-term Debt [Line Items]
Weighted average borrowings outstanding	$ 273,900,000	$ 300,600,000
Weighted average interest rate	0.90%	0.70%
Minimum borrowing outstanding	$ 73,100,000	$ 180,100,000
Maximum borrowing outstanding	427,200,000	488,500,000
Short-term debt	$ 398,700,000	$ 338,000,000
Weighted average interest rate	1.10%	0.80%
Laclede Gas
Short-term Debt [Line Items]
Weighted average borrowings outstanding	$ 215,700,000	$ 213,000,000
Weighted average interest rate	0.70%	0.40%
Minimum borrowing outstanding	$ 127,800,000.0	$ 104,200,000.0
Maximum borrowing outstanding	307,200,000.0	341,000,000
Short-term debt	$ 243,700,000	$ 233,000,000
Weighted average interest rate	0.80%	0.50%
Laclede Gas | Commercial Paper
Short-term Debt [Line Items]
Weighted average borrowings outstanding	$ 201,000,000	$ 212,700,000
Weighted average interest rate	0.70%	0.40%
Minimum borrowing outstanding	$ 43,000,000	$ 102,100,000.0
Maximum borrowing outstanding	307,200,000.0	341,000,000
Short-term debt	$ 243,700,000	$ 233,000,000
Weighted average interest rate	0.80%	0.50%
Laclede Gas | Notes Payable to Related Party
Short-term Debt [Line Items]
Weighted average borrowings outstanding	$ 14,700,000	$ 300,000
Weighted average interest rate	0.80%	0.50%
Minimum borrowing outstanding	$ 0	$ 0
Maximum borrowing outstanding	114,200,000.0	10,400,000.0
Short-term debt	$ 0	$ 0
Weighted average interest rate	0.00%	0.00%