Shareholders' Equity - Additional Information (Details) - USD ($)			12 Months Ended
	May 21, 2019	Feb. 06, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Nov. 13, 2020	Mar. 24, 2020
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.
Sale of common stock			$ 32,000,000.0	14,400,000	$ 153,000,000.0
Issuance of common stock			$ 41,100,000	19,500,000	$ 154,700,000
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)			207,254
Dividend payment restriction formula component - fixed amount			$ 8,000,000.0
Retained earnings available to pay dividends in period			$ 1,269,400,000	$ 1,182,400,000
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			$ 500,000,000.0
Regulatory preferred stock and debt securities issuance, remaining authorized amount			$ 125,000,000.0
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
Regulatory preferred stock and debt securities issuance, amount authorized							$ 150,000,000.0
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)						201,148
ATM Program
Class Of Stock [Line Items]
Sale of common stock		$ 150,000,000.0
Number of shares issued			446,619	179,630
Issuance of common stock			$ 32,000,000.0	$ 14,400,000
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