Notes Payable and Credit Agreements - Additional Information (Details)	12 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 14, 2016 bank
Line of Credit Facility [Line Items]
Long-term debt	$ 975,000,000.0
Credit facility borrowings	0	$ 0
Lender Concentration Risk | Liabilities
Line of Credit Facility [Line Items]
Line of credit facility, number of banks in agreement | bank			11
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	0.53
Commercial Paper Notes
Line of Credit Facility [Line Items]
Line of credit facility borrowing capacity	$ 975,000,000.0
Debt term	365 days
Spire Missouri | Revolving Credit Facility Borrowings
Line of Credit Facility [Line Items]
Line of credit facility borrowing capacity	$ 475,000,000.0
Debt to total capitalization ratio	0.50
Spire Alabama | Revolving Credit Facility Borrowings
Line of Credit Facility [Line Items]
Line of credit facility borrowing capacity	$ 200,000,000.0
Debt to total capitalization ratio	0.40
Spire
Line of Credit Facility [Line Items]
Short-term borrowings used to support lending to Utilities	$ 529,900,000
Spire | Revolving Credit Facility Borrowings
Line of Credit Facility [Line Items]
Line of credit facility borrowing capacity	$ 300,000,000.0