Shareholders' Equity - Additional Information (Details) - USD ($)				1 Months Ended		3 Months Ended
	Mar. 05, 2024	Dec. 11, 2023	May 21, 2019	Dec. 31, 2024	Feb. 28, 2021	Sep. 30, 2024	Oct. 23, 2025	Sep. 30, 2025	Mar. 31, 2025	Mar. 01, 2024	Jan. 25, 2024	Sep. 30, 2022
Class of Stock [Line Items]
Preferred Stock, Shares Authorized (in shares)						5,000,000		5,000,000
Number of Shares Authorized for Issuance (in shares)								250,000
Number Of Shares Remaining and Available for Issuance (in shares)						218,141		196,135
Retained Earnings Dividend Restrictions Formula Component Fixed Amount												$ 8,000,000
Retained Earnings, Unappropriated						$ 1,797,000,000		$ 1,999,400,000
Common stock						$ 57,700,000		$ 59,000,000
Spire Missouri [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Authorized (in shares)						1,480,000		1,480,000
Regulatory Preferred Stock and Debt Securities Issuance Amount Authorized								$ 850,000,000
Preferred Stock, Shares Issued (in shares)						0		0
Common stock						$ 100,000		$ 100,000
Spire Missouri [Member] | Subsequent Event [Member]
Class of Stock [Line Items]
Common stock							$ 74,400,000
Spire Alabama Inc [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Authorized (in shares)						120,000		120,000
Preferred Stock, Shares Issued (in shares)						0		0
At The Market Program [Member]
Class of Stock [Line Items]
Forward Sales Agreements, Shares Settled (in shares)		1,744,549		542,515
Proceeds from Shares Settled		$ 112,200,000		$ 32,400,000
Sale of Stock, Maximum Aggregate Offering Price								$ 123,600,000			$ 200,000,000
Forward Sales Agreements, Shares (in shares)						663,619
Forward Sales Agreements, Proceeds If Settled									$ 42,400,000
First Mortgage Bond [Member] | Spire Missouri [Member] | Subsequent Event [Member]
Class of Stock [Line Items]
Debt Instrument face amount							350,000,000
Parent Company [Member] | Two Thousand Twenty One Series A Zero Point Seven Five Percentage Remarketable Senior Notes [Member]
Class of Stock [Line Items]
Debt Instrument face amount					$ 1,000
Debt Instrument, Interest Rate, Stated Percentage					0.75%
Parent Company [Member] | First Mortgage Bond [Member] | Subsequent Event [Member]
Class of Stock [Line Items]
Debt Instrument face amount							$ 150,000,000
Depositary Shares [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Authorized (in shares)						10,000,000		10,000,000
Preferred Stock, Shares Outstanding, Ending Balance (in shares)			10,000,000			10,000,000		10,000,000
Fractional Interest In Share Of Series A Preferred Stock			0.001%
Preferred Stock, Par or Stated Value Per Share (in dollars per share)						$ 25		$ 25
Preferred Stock, Liquidation Preference Per Share (in dollars per share)			$ 25
Preferred Stock, Shares Issued (in shares)						10,000,000		10,000,000
Series A Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Dividend Rate, Percentage			5.90%
Preferred Stock, Par or Stated Value Per Share (in dollars per share)			$ 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
Corporate Unit [Member] | Parent Company [Member]
Class of Stock [Line Items]
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