Note 5 - Shareholders' Equity (Tables)

12 Months Ended

Sep. 30, 2023

Notes Tables

Stock Purchase Contract Obligation, Number of Shares to be Purchased [Table Text Block]

If the applicable market value* per share	Number of shares to be purchased per
of Spire common stock is:	stock purchase contract is:
Equal to or greater than $78.6906 (“threshold appreciation price”)	0.6354 (“minimum settlement rate”)
Less than $78.6906, but greater than $64.24	$50.00 ÷ applicable market value*
Less than or equal to $64.24 (“reference price”)	0.7783 (“maximum settlement rate”)

Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block]

	Net		Defined Benefit			Net Unrealized
	Unrealized		Pension and			Gain (Loss) on
	Gain (Loss)		Other			Available-for-
	on Cash Flow		Postretirement			Sale Debt
	Hedges		Benefit Plans			Securities			Total
Spire
Balance at September 30, 2021	$	7.9	$	(4.2	)	$	(0.1	)	$	3.6
Other comprehensive income (loss)		42.5		1.5			(0.4	)		43.6
Balance at September 30, 2022		50.4		(2.7	)		(0.5	)		47.2
Other comprehensive income		0.1		0.2			0.1			0.4
Balance at September 30, 2023	$	50.5	$	(2.5	)	$	(0.4	)	$	47.6
Spire Missouri
Balance at September 30, 2021	$	—	$	(4.2	)	$	—		$	(4.2	)
Other comprehensive income		—		1.5			—			1.5
Balance at September 30, 2022		—		(2.7	)		—			(2.7	)
Other comprehensive income		—		0.2			—			0.2
Balance at September 30, 2023	$	—	$	(2.5	)	$	—		$	(2.5	)