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Financing - Additional Information (Details)				1 Months Ended	3 Months Ended		6 Months Ended
Financing - Additional Information (Details)	Mar. 23, 2021 USD ($)	Dec. 15, 2020 USD ($)	Mar. 26, 2020 USD ($) bank	Feb. 28, 2021 USD ($) $ / shares shares	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($) shares	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($) shares	Dec. 14, 2016 bank
Debt Instrument And Equity Units [Line Items]
Long-term Debt					$ 975,000,000.0		$ 975,000,000.0
Common stock issued						$ 9,500,000		$ 9,500,000
Common Stock $1.00 par value
Debt Instrument And Equity Units [Line Items]
Common stock issued (in shares) \| shares						112,758		112,758
Common stock issued						$ 100,000		$ 100,000
Liabilities \| Lender Concentration Risk
Debt Instrument And Equity Units [Line Items]
Line of credit facility, number of banks in agreement \| bank									11
Spire
Debt Instrument And Equity Units [Line Items]
Term loan bear interest rate per annum							0.40%
Short-term borrowings used to support lending to Utilities					398,100,000		$ 398,100,000
Present value of stock purchase contract payment recorded as liability					35,000,000.0
Spire \| Common Stock $1.00 par value
Debt Instrument And Equity Units [Line Items]
Price divided by applicable market value to determine number of shares per stock purchase contract \| $ / shares				$ 50.00
Spire \| 2021 Series A 0.75% Remarketable Senior Notes
Debt Instrument And Equity Units [Line Items]
Stated interest rate				0.75%
Debt instrument, maturity date				Mar. 01, 2026
Debt instrument, payment terms				The Company will make quarterly interest payments on the RSNs at the rate of 0.75% per year and quarterly contract adjustment payments on the stock purchase contracts at the rate of 6.75%.
Common stock issued (in shares) \| shares				3,500,000
Common stock issued				$ 175,000,000.0
Net proceeds after underwriting fees and other issuance costs				$ 169,300,000
Units price \| $ / shares				$ 50
Percentage of undivided beneficial ownership interest				5.00%
Principal amount				$ 1,000
Stock purchase contracts rate				6.75%
Spire \| 2021 Series A 0.75% Remarketable Senior Notes \| Stock Purchase Contract
Debt Instrument And Equity Units [Line Items]
Units price \| $ / shares				$ 50
Spire \| Term Loan
Debt Instrument And Equity Units [Line Items]
Line of credit facility, number of banks in agreement \| bank			2
Long-term Debt			$ 150,000,000.0
Team loan interest rate description							The term loan bore interest at the LIBOR Rate (as defined in the loan agreement) plus 0.85% per annum.
Term loan bear interest rate per annum							1.10%
Spire \| Term Loan \| LIBOR
Debt Instrument And Equity Units [Line Items]
Term loan bear interest rate per annum			0.85%
Spire \| Senior Notes \| 2021 Series A 0.75% Remarketable Senior Notes
Debt Instrument And Equity Units [Line Items]
Debt amount				$ 175,000,000.0
Stated interest rate				0.75%
Spire \| Maximum \| Common Stock $1.00 par value
Debt Instrument And Equity Units [Line Items]
Number of shares to be purchased per stock purchase contract, settlement rate \| shares				0.7783
Spire \| Minimum \| Common Stock $1.00 par value
Debt Instrument And Equity Units [Line Items]
Number of shares to be purchased per stock purchase contract, settlement rate \| shares				0.6354
Spire Missouri \| Term Loan
Debt Instrument And Equity Units [Line Items]
Term loan bear interest rate per annum							0.80%
Spire Alabama \| Senior Notes \| Private Placement \| Series 2020 Senior Notes due 2030
Debt Instrument And Equity Units [Line Items]
Long-term debt		$ 150,000,000.0
Stated interest rate		2.04%
Debt instrument, maturity date		Dec. 15, 2030
Debt instrument, payment terms							Interest is payable semi-annually.
Revolving Credit Facility Borrowings
Debt Instrument And Equity Units [Line Items]
Line of credit facility borrowing capacity					$ 975,000,000.0		$ 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%		70.00%
Revolving Credit Facility Borrowings \| Maximum
Debt Instrument And Equity Units [Line Items]
Debt to total capitalization ratio					0.60		0.60
Revolving Credit Facility Borrowings \| Spire
Debt Instrument And Equity Units [Line Items]
Line of credit facility borrowing capacity					$ 300,000,000.0		$ 300,000,000.0
Revolving Credit Facility Borrowings \| Spire Missouri
Debt Instrument And Equity Units [Line Items]
Line of credit facility borrowing capacity					475,000,000.0		475,000,000.0
Revolving Credit Facility Borrowings \| Spire Alabama
Debt Instrument And Equity Units [Line Items]
Line of credit facility borrowing capacity					200,000,000.0		200,000,000.0
Commercial Paper Notes
Debt Instrument And Equity Units [Line Items]
Debt amount					$ 975,000,000.0		$ 975,000,000.0
Debt term							365 days
Commercial Paper Notes \| Spire
Debt Instrument And Equity Units [Line Items]
Term loan bear interest rate per annum							0.30%
Unsecured Term Loan \| Spire Missouri
Debt Instrument And Equity Units [Line Items]
Debt amount	$ 250,000,000.0
Debt term	364 days
Prepayment penalty	$ 0
Unsecured Term Loan \| Spire Missouri \| LIBOR
Debt Instrument And Equity Units [Line Items]
Interest rate	0.65%