Note 1 - Summary of Significant Accounting Policies (Tables)

12 Months Ended

Sep. 30, 2023

Notes Tables

Schedule Of Capital Expenditure Excluded From Statement Of Cash Flow [Table Text Block]

September 30	2023		2022		2021
Spire	$	104.3	$	77.8	$	59.5
Spire Missouri		56.5		45.6		37.1
Spire Alabama		4.6		19.2		13.6

Schedule of Asset Retirement Obligations [Table Text Block]

	Spire						Spire Missouri						Spire Alabama
	2023			2022			2023			2022			2023			2022
Asset retirement obligations, beginning of year	$	520.9		$	519.6		$	110.6		$	143.4		$	398.7		$	362.8
Liabilities incurred during the period		7.9			3.2			0.9			1.2			2.0			0.5
Liabilities settled during the period		(6.8	)		(9.2	)		(2.5	)		(4.1	)		(2.5	)		(2.0	)
Accretion		21.5			21.1			4.5			5.9			16.5			14.7
Revisions in estimated cash flows		33.9			(13.8	)		(2.4	)		(35.8	)		36.3			22.7
Asset retirement obligations, end of year	$	577.4		$	520.9		$	111.1		$	110.6		$	451.0		$	398.7

Schedule of Related Party Transactions [Table Text Block]

	Spire Missouri						Spire Alabama
	2023		2022		2021		2023		2022		2021
Natural gas purchases from Spire Marketing	$	57.4	$	86.3	$	92.5	$	4.7	$	3.2	$	10.4
Natural gas sales to Spire Marketing		0.4		—		1.1		—		0.5		0.1
Transportation services from Spire STL Pipeline LLC		32.0		32.0		32.0		—		—		—
Natural gas storage services from Spire Storage Salt Plains LLC		0.7		—		—		—		—		—
Transportation services from Spire NGL Inc.		—		—		0.5		—		—		—

Accounts Receivable, Allowance for Credit Loss [Table Text Block]

	Spire									Spire Missouri									Spire Alabama
	2023			2022			2021			2023			2022			2021			2023			2022			2021
Allowance at beginning of year	$	31.9		$	30.3		$	24.9		$	24.9		$	22.6		$	18.1		$	6.3		$	6.6		$	5.5
Provision for expected credit losses		16.6			11.6			14.7			13.3			11.2			11.1			2.6			0.3			3.1
Write-offs, net of recoveries		(16.0	)		(10.0	)		(9.3	)		(12.0	)		(8.9	)		(6.6	)		(3.2	)		(0.6	)		(2.0	)
Allowance at end of year	$	32.5		$	31.9		$	30.3		$	26.2		$	24.9		$	22.6		$	5.7		$	6.3		$	6.6