Note 5 - Shareholders' Equity (Details Textual) - USD ($)				1 Months Ended	3 Months Ended			12 Months Ended
	Mar. 05, 2024	Dec. 11, 2023	May 21, 2019	Feb. 28, 2021	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Sep. 30, 2024	Mar. 01, 2024	Jan. 25, 2024	Sep. 30, 2023	Sep. 30, 2022
Preferred Stock, Shares Authorized (in shares)					5,000,000			5,000,000			5,000,000
Number of Shares Authorized for Issuance (in shares)					250,000			250,000
Number Of Shares Remaining and Available for Issuance (in shares)					218,141			218,141			244,182
Retained Earnings Dividend Restrictions Formula Component Fixed Amount												$ 8,000,000
Retained Earnings, Unappropriated					$ 1,797,000,000			$ 1,797,000,000			$ 1,678,500,000
Spire Missouri [Member]
Preferred Stock, Shares Authorized (in shares)					1,480,000			1,480,000			1,480,000
Preferred Stock, Shares Outstanding, Ending Balance (in shares)					0			0			0
Regulatory Preferred Stock and Debt Securities Issuance Amount Authorized					$ 850,000,000			$ 850,000,000
Preferred Stock, Shares Issued (in shares)					0			0			0
Spire Alabama Inc [Member]
Preferred Stock, Shares Authorized (in shares)					120,000			120,000			120,000
Preferred Stock, Shares Outstanding, Ending Balance (in shares)					0			0			0
Preferred Stock, Shares Issued (in shares)					0			0			0
At The Market Program [Member]
Forward Sales Agreements, Shares Settled (in shares)		1,744,549						0
Proceeds from Shares Settled		$ 112,200,000
Sale of Stock, Maximum Aggregate Offering Price					$ 123,600,000			$ 123,600,000		$ 200,000,000
Forward Sales Agreements, Shares (in shares)					663,619	338,110	204,405
Forward Sales Agreements, Proceeds If Settled					$ 75,000,000			$ 75,000,000
Parent Company [Member] | Two Thousand Twenty One Series A Zero Point Seven Five Percentage Remarketable Senior Notes [Member]
Debt Instrument, Face Amount				$ 1,000
Debt Instrument, Interest Rate, Stated Percentage				0.75%
Depositary Shares [Member]
Preferred Stock, Shares Authorized (in shares)					10,000,000			10,000,000			10,000,000
Preferred Stock, Shares Outstanding, Ending Balance (in shares)			10,000,000		10,000,000			10,000,000			10,000,000
Fractional Interest In Share Of Series A Preferred Stock			0.001%
Preferred Stock, Liquidation Preference Per Share (in dollars per share)			$ 25
Preferred Stock, Shares Issued (in shares)					10,000,000			10,000,000			10,000,000
Series A Preferred Stock [Member]
Preferred Stock, Dividend Rate, Percentage			5.90%
Preferred Stock, Par or Stated Value Per Share (in dollars per share)			$ 25		$ 25			$ 25			$ 25
Preferred Stock, Liquidation Preference Per Share (in dollars per share)			$ 25,000
Stock Issued During Period, Shares, New Issues (in shares)			10,000
Proceeds from Issuance of Preferred Stock and Preference Stock			$ 242,000,000
Preferred Stock, Redemption Price Per Share (in dollars per share)					$ 25,000			$ 25,000
Corporate Unit [Member] | Parent Company [Member]
Stock Issued During Period, Shares, New Issues (in shares)	2,745,733			3,500,000
Equity Units, Stated Value (in dollars per share)				$ 50
Percentage of Undivided Beneficial Ownership Interest				5.00%
Equity Units, Applicable Market Value (in dollars per share)									$ 58.6809
Equity Units, Number of Common Stock Obligated to be Purchased After Adjustment (in shares)									0.7845
Equity Stock Units Issued	$ 175,000,000