Shareholders' Equity - Additional Information (Details) - USD ($)			1 Months Ended	12 Months Ended
	May 21, 2019	Feb. 06, 2019	Feb. 28, 2021	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019	Nov. 12, 2021
Class Of Stock [Line Items]
Preferred stock shares authorized (in shares)				5,000,000	5,000,000
Percentage of preferred stock	5.90%
Preferred stock, liquidation preference per share	$ 25,000
Net proceeds, after deducting commissions and sale expenses						$ 242,000,000.0
Preferred stock, redemption date				Aug. 15, 2024
Preferred stock voting rights				Shareholders of the Preferred Stock generally have no voting rights with respect to matters that generally require the approval of voting stockholders. The limited voting rights of holders of the Preferred Stock include the right to vote on certain matters that may affect the preference or special rights of the Preferred Stock. In addition, if and whenever dividends on any shares of Preferred Stock have not been declared and paid for at least six dividend periods, whether or not consecutive, the number of directors then constituting the Board shall automatically be increased by two (to be elected by the holders of the Preferred Stock) until all accumulated and unpaid dividends on the Preferred Stock have been paid in full.
Aggregate stated amount of resulting net proceeds				$ (40,800,000)
Sale of common stock					$ 32,000,000.0	$ 14,400,000
SEC effective shelf registration Form S-3 sale and issuance authorization under the dividend reinvestment and stock purchase program (in shares)				250,000
SEC effective shelf registration Form S-3 remaining and available for issuance under the dividend reinvestment and stock purchase program (in shares)				182,689
Dividend payment restriction formula component - fixed amount				$ 8,000,000.0
Retained earnings available to pay dividends in period				$ 1,413,400,000	$ 1,269,400,000
Spire Missouri
Class Of Stock [Line Items]
Preferred stock shares authorized (in shares)				1,480,000	1,480,000
Preferred stock shares outstanding (in shares)				0	0
Regulatory preferred stock and debt securities issuance, amount authorized				$ 660,000,000.0
Regulatory preferred stock and debt securities issuance, remaining authorized amount				$ 355,000,000.0
Preferred stock shares issued (in shares)				0	0
Spire Alabama
Class Of Stock [Line Items]
Preferred stock shares authorized (in shares)				120,000	120,000
Preferred stock shares outstanding (in shares)				0	0
Preferred stock shares issued (in shares)				0	0
Subsequent Event
Class Of Stock [Line Items]
SEC effective shelf registration Form S-3 remaining and available for issuance under the dividend reinvestment and stock purchase program (in shares)							177,295
ATM Program
Class Of Stock [Line Items]
Number of shares issued				626,249
Sale of common stock		$ 150,000,000.0		$ 102,200,000
Common Stock
Class Of Stock [Line Items]
Number of shares issued					446,619	179,630
Sale of common stock					$ 400,000	$ 200,000
Spire
Class Of Stock [Line Items]
Present value of stock purchase contract payment recorded as liability				$ 35,000,000.0
Spire | Common Stock
Class Of Stock [Line Items]
Price divided by applicable market value to determine number of shares per stock purchase contract			$ 50.00
Spire | Common Stock | Minimum [Member]
Class Of Stock [Line Items]
Number of shares to be purchased per stock purchase contract, settlement rate			0.6354
Spire | Common Stock | Maximum
Class Of Stock [Line Items]
Number of shares to be purchased per stock purchase contract, settlement rate			0.7783
Spire | Two Thousand Twenty One Series A Zero Point Seven Five Percentage Remarketable Senior Notes
Class Of Stock [Line Items]
Number of shares issued			3,500,000
Aggregate stated amount of resulting net proceeds			$ 175,000,000.0
Net proceeds after underwriting fees and other issuance costs			$ 169,300,000
Units price			$ 50
Percentage of undivided beneficial ownership interest			5.00%
Principal amount			$ 1,000
Stated interest rate			0.75%
Debt instrument, maturity date			Mar. 01, 2026
Stock purchase contracts rate			6.75%
Debt instrument, payment terms			The Company makes 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%.
Spire | Stock Purchase Contract | Two Thousand Twenty One Series A Zero Point Seven Five Percentage Remarketable Senior Notes
Class Of Stock [Line Items]
Units price			$ 50
Redemption prior to August 15, 2024
Class Of Stock [Line Items]
Preferred stock redemption price per share				$ 25,500
Redemption on or after August 15, 2024
Class Of Stock [Line Items]
Preferred stock redemption price per share				$ 25,000
Depositary Shares
Class Of Stock [Line Items]
Preferred stock shares authorized (in shares)				10,000,000.0	10,000,000.0
Preferred stock shares outstanding (in shares)	10,000,000			10,000,000.0	10,000,000.0
Preferred stock share interest rate	0.10%
Preferred stock, par value	$ 25.00			$ 25.00	$ 25.00
Net proceeds, after deducting commissions and sale expenses	$ 242,000,000.0
Preferred stock shares issued (in shares)				10,000,000.0	10,000,000.0