NOTES PAYABLE AND CREDIT AGREEMENTS - Spire Missouri and Alabama Short-term Borrowings (Details) - USD ($)	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Short-term Debt [Line Items]
Weighted average borrowings outstanding	$ 408,600,000
Weighted average interest rate	2.00%
Minimum borrowing outstanding	$ 146,000,000
Maximum borrowing outstanding	632,900,000
Borrowings outstanding	$ 553,600,000	$ 477,300,000
Weighted average interest rate	2.40%	1.50%
Spire Missouri | Borrowings
Short-term Debt [Line Items]
Weighted average borrowings outstanding	$ 210,500,000
Weighted average interest rate	2.10%
Minimum borrowing outstanding	$ 101,500,000.0
Maximum borrowing outstanding	345,300,000.0
Borrowings outstanding	$ 345,300,000	$ 203,000,000
Weighted average interest rate	2.30%	1.50%
Spire Alabama | Borrowings
Short-term Debt [Line Items]
Weighted average borrowings outstanding	$ 103,900,000
Weighted average interest rate	2.00%
Minimum borrowing outstanding	$ 33,600,000.0
Maximum borrowing outstanding	188,600,000.0
Borrowings outstanding	$ 142,500,000	$ 169,900,000
Weighted average interest rate	2.30%	1.50%