NOTES PAYABLE AND CREDIT AGREEMENTS - Narrative (Details)	12 Months Ended
	Sep. 30, 2018 USD ($)	Dec. 14, 2016 bank
Spire Missouri
Line of Credit Facility [Line Items]
Long-term debt	$ 975,000,000
Debt term	365 days
Revolving Credit Facility Borrowings
Line of Credit Facility [Line Items]
Line of credit facility borrowing capacity	$ 975,000,000.0
Minimum required ratio of earnings before interest, taxes, depreciation, and amortization (EBITDA) times interest expense as required by the line of credit covenant	70.00%
Debt to total capitalization ratio	0.54
Revolving Credit Facility Borrowings | Spire Missouri
Line of Credit Facility [Line Items]
Line of credit facility borrowing capacity	$ 475,000,000.0
Debt to total capitalization ratio	0.49
Revolving Credit Facility Borrowings | Spire Alabama
Line of Credit Facility [Line Items]
Line of credit facility borrowing capacity	$ 200,000,000.0
Debt to total capitalization ratio	0.37
Revolving Credit Facility Borrowings | Liabilities | Lender Concentration Risk
Line of Credit Facility [Line Items]
Line of credit facility, number of banks in agreement | bank		11
Spire | Revolving Credit Facility Borrowings
Line of Credit Facility [Line Items]
Line of credit facility borrowing capacity	$ 300,000,000.0
Subsidiaries
Line of Credit Facility [Line Items]
Notes outstanding	$ 540,500,000