NOTES PAYABLE AND CREDIT AGREEMENTS - Alagasco's Short-term Borrowings (Details) - USD ($)	1 Months Ended	12 Months Ended
	Sep. 30, 2014	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 Alabama
Short-term Debt [Line Items]
Weighted average borrowings outstanding		$ 106,900,000	$ 42,600,000
Weighted average interest rate		1.50%	1.40%
Minimum borrowing outstanding		$ 74,000,000	$ 19,000,000
Maximum borrowing outstanding		171,000,000	82,000,000
Short-term debt		$ 169,900,000	$ 82,000,000
Weighted average interest rate		1.50%	1.50%
Spire Alabama | Line of Credit
Short-term Debt [Line Items]
Weighted average borrowings outstanding	$ 30,200,000	$ 28,300,000	$ 30,200,000
Weighted average interest rate	1.40%	1.60%	1.40%
Minimum borrowing outstanding		$ 0	$ 0
Maximum borrowing outstanding		102,500,000.0	82,000,000.0
Short-term debt		$ 0	$ 82,000,000
Weighted average interest rate		0.00%	1.50%
Spire Alabama | Notes Payable to Related Party
Short-term Debt [Line Items]
Weighted average borrowings outstanding		$ 78,600,000	$ 12,400,000
Weighted average interest rate		1.40%	1.40%
Minimum borrowing outstanding		$ 0	$ 0
Maximum borrowing outstanding		171,000,000.0	61,900,000.0
Short-term debt		$ 169,900,000	$ 0
Weighted average interest rate		1.50%	0.00%