Shareholder's Equity - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions			3 Months Ended		9 Months Ended
	May 21, 2019	Feb. 06, 2019	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Class Of Stock [Line Items]
Preferred stock shares			10,000,000.0		10,000,000.0
Percentage of preferred stock	5.90%
Preferred stock, par value			$ 25.00		$ 25.00
Preferred stock, liquidation preference per share	$ 25,000
Net proceeds, after deducting commissions and sale expenses					$ 242.0	$ 0.0
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 Spire’s Board of Directors 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			$ 4.5	$ 153.0	$ 4.5	153.0
Issuance of common stock					$ 5.7	$ 154.2
ATM Program
Class Of Stock [Line Items]
Sale of common stock		$ 150.0
Number of shares issued			59,630
Issuance of common stock			$ 4.9
Redemption prior to August 15, 2024
Class Of Stock [Line Items]
Preferred stock redemption price per share			$ 25,500		$ 25,500
Redemption on or after August 15, 2024
Class Of Stock [Line Items]
Preferred stock redemption price per share			$ 25,000		$ 25,000
Depositary Shares
Class Of Stock [Line Items]
Preferred stock shares	10,000,000
Preferred stock share interest rate	0.10%
Preferred stock, par value	$ 25.00
Net proceeds, after deducting commissions and sale expenses	$ 242.0