NOTES PAYABLE AND CREDIT AGREEMENTS - Laclede Gas' Short-term Borrowings (Details) - USD ($)	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Short-term Debt [Line Items]
Weighted average borrowings outstanding	$ 485,800,000	$ 273,900,000
Weighted average interest rate	1.20%	0.90%
Minimum borrowing outstanding	$ 395,500,000	$ 73,100,000
Maximum borrowing outstanding	675,600,000	427,200,000
Short-term debt	$ 477,300,000	$ 398,700,000
Weighted average interest rate	1.50%	1.10%
Spire Missouri
Short-term Debt [Line Items]
Weighted average borrowings outstanding	$ 284,000,000	$ 215,700,000
Weighted average interest rate	1.20%	0.70%
Minimum borrowing outstanding	$ 168,300,000.0	$ 127,800,000.0
Maximum borrowing outstanding	358,900,000.0	307,200,000.0
Short-term debt	$ 203,000,000	$ 243,700,000
Weighted average interest rate	1.50%	0.80%
Spire Missouri | Commercial Paper
Short-term Debt [Line Items]
Weighted average borrowings outstanding	$ 88,500,000	$ 201,000,000
Weighted average interest rate	0.90%	0.70%
Minimum borrowing outstanding	$ 0	$ 43,000,000.0
Maximum borrowing outstanding	329,700,000.0	307,200,000.0
Short-term debt	$ 0	$ 243,700,000
Weighted average interest rate	0.00%	0.80%
Spire Missouri | Notes Payable to Related Party
Short-term Debt [Line Items]
Weighted average borrowings outstanding	$ 195,500,000	$ 14,700,000
Weighted average interest rate	1.30%	0.80%
Minimum borrowing outstanding	$ 0	$ 0
Maximum borrowing outstanding	338,600,000.0	114,200,000.0
Short-term debt	$ 203,000,000	$ 0
Weighted average interest rate	1.50%	0.00%