Note 5 - Shareholders' Equity (Details Textual) - USD ($)			1 Months Ended	3 Months Ended		12 Months Ended		19 Months Ended
	Sep. 27, 2023	May 21, 2019	Feb. 28, 2021	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Mar. 01, 2024	Nov. 10, 2023	Apr. 28, 2022
Preferred Stock, Shares Authorized (in shares)						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)						244,182		244,182
Retained Earnings Dividend Restrictions Formula Component Fixed Amount							$ 8,000,000
Retained Earnings, Unappropriated						$ 1,678,500,000	1,579,400,000	$ 1,678,500,000
Stock Issued During Period, Value, New Issues						$ 40,500,000	$ 50,300,000
Spire Missouri [Member]
Preferred Stock, Shares Authorized (in shares)							1,480,000
Preferred Stock, Shares Outstanding, Ending Balance (in shares)						0	0	0
Regulatory Preferred Stock and Debt Securities Issuance Amount Authorized						$ 800,000,000		$ 800,000,000
Stock Issued During Period, Value, New Issues						$ 38,700,000	$ 51,100,000
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
Forecast [Member]
Number Of Shares Remaining and Available for Issuance (in shares)										237,891
At The Market Program [Member]
Number of Shares Authorized for Issuance (in shares)						17,900,000		17,900,000			200,000,000
Forward Sales Agreements, Shares (in shares)				2,087,231	228,690
Forward Sales Agreements, Shares Settled (in shares)	571,372
Proceeds from Shares Settled	$ 38,500,000
Forward Sales Agreements, Block Sales, Shares (in shares)						1,744,549
Forward Sales Agreements, Proceeds If Settled						$ 111,200,000		$ 111,200,000
Universal Shelf Registration [Member] | Spire Missouri [Member]
Stock Issued During Period, Value, New Issues						$ 79,100,000		79,100,000
Common Stock [Member]
Stock Issued During Period, Shares, New Issues (in shares)						611,872	719,625
Stock Issued During Period, Value, New Issues						$ 600,000	$ 700,000
Common Stock [Member] | Spire Missouri [Member]
Stock Issued During Period, Shares, New Issues (in shares)						530	748
Stock Issued During Period, Value, New Issues						$ 0	$ 0
First Mortgage Bond [Member] | Spire Missouri [Member]
Debt Instrument, Face Amount						400,000,000		400,000,000
Parent Company [Member]
Present Value of Stock Purchase Contract Payment						$ 35,000,000		$ 35,000,000
Parent Company [Member] | Common Stock [Member] | Minimum [Member]
Number of Shares to Be Purchased Per Stock Purchase Contract Settlement Rate (in shares)			0.6354
Parent Company [Member] | Common Stock [Member] | Minimum [Member] | Forecast [Member]
Number of Shares to Be Purchased Per Stock Purchase Contract Settlement Rate (in shares)									0.6354
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%
Stock Purchase Contracts Rate			6.75%
Redemption Prior to August Fifteen Two Thousand Twenty Four [Member]
Preferred Stock, Redemption Price Per Share (in dollars per share)						$ 25,500		$ 25,500
Redemption on Or After August Fifteen Two Thousand Twenty Four [Member]
Preferred Stock, Redemption Price Per Share (in dollars per share)						$ 25,000		$ 25,000
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
Preferred Stock, Share Interest Rate		0.001%
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
Proceeds from Issuance of Preferred Stock and Preference Stock		$ 242,000,000
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%
Corporate Unit [Member] | Parent Company [Member]
Stock Issued During Period, Shares, New Issues (in shares)			3,500,000
Equity Stock Units Issued			$ 175,000,000
Proceeds from Issuance or Sale of Equity, Total			$ 169,300,000
Equity Units, Stated Value (in dollars per share)			$ 50
Percentage of Undivided Beneficial Ownership Interest			5.00%
Shares Issued, Price Per Share (in dollars per share)			$ 50