Financing Arrangements and Long-term Debt - Additional Information (Details)	1 Months Ended		6 Months Ended
	Dec. 31, 2018 USD ($)	Oct. 31, 2018 USD ($)	Mar. 31, 2019 USD ($)	Jan. 15, 2019 USD ($)	Sep. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 14, 2016 bank
Line of Credit Facility [Line Items]
Line of credit facility borrowing capacity			$ 975,000,000
Credit facility borrowings			0		$ 0	$ 0
Non-interest-bearing Note
Line of Credit Facility [Line Items]
Settlement of debt		$ 10,000,000
Term Loan
Line of Credit Facility [Line Items]
Long-term Debt	$ 100,000,000
Debt instrument, maturity date	Dec. 01, 2021
Liabilities | Lender Concentration Risk
Line of Credit Facility [Line Items]
Line of credit facility, number of banks in agreement | bank							11
Spire
Line of Credit Facility [Line Items]
Line of credit facility borrowing capacity			300,000,000
Short-term borrowings used to support lending to Utilities			344,600,000
Spire Missouri
Line of Credit Facility [Line Items]
Line of credit facility borrowing capacity			475,000,000
Spire Alabama
Line of Credit Facility [Line Items]
Line of credit facility borrowing capacity			$ 200,000,000
Spire Alabama | Senior Notes | Private Placement | Series 2019 A senior notes due 2049
Line of Credit Facility [Line Items]
Long-term debt				$ 90,000,000
Stated interest rate				4.64%
Line of Credit
Line of Credit Facility [Line Items]
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
Line of Credit | Spire Missouri
Line of Credit Facility [Line Items]
Debt to total capitalization ratio			0.47
Line of Credit | Spire Alabama
Line of Credit Facility [Line Items]
Debt to total capitalization ratio			0.37
Commercial Paper Notes
Line of Credit Facility [Line Items]
Long-term debt			$ 975,000,000
Debt term			365 days