Pension Plans and Other Postretirement Benefits (Tables)

12 Months Ended

Sep. 30, 2025

Retirement Benefits [Abstract]

Schedule of Net Benefit Costs

	Spire									Spire Missouri									Spire Alabama
	2025			2024			2023			2025			2024			2023			2025			2024			2023
Service cost – benefits earned during    the period	$	17.6		$	15.7		$	16.3		$	11.4		$	10.4		$	11.4		$	5.5		$	4.6		$	4.2
Interest cost on projected benefit    obligation		23.5			26.7			25.1			15.8			18.9			17.8			5.1			5.1			4.8
Expected return on plan assets		(25.9	)		(24.8	)		(24.5	)		(17.6	)		(17.0	)		(18.0	)		(5.3	)		(4.8	)		(3.5	)
Amortization of prior service credit		(4.5	)		(4.5	)		(4.5	)		(1.9	)		(1.9	)		(1.9	)		(2.4	)		(2.4	)		(2.4	)
Amortization of actuarial loss		6.5			6.4			6.6			4.1			5.9			6.1			2.4			1.0			0.9
Special termination benefits		—			2.6			—			—			2.6			—			—			—			—
Loss on lump-sum settlements and    curtailments		0.4			12.9			9.7			0.4			12.9			0.6			—			—			9.1
Subtotal		17.6			35.0			28.7			12.2			31.8			16.0			5.3			3.5			13.1
Regulatory adjustment		36.3			18.4			26.3			29.6			10.8			27.1			5.8			6.7			(1.7	)
Net pension cost	$	53.9		$	53.4		$	55.0		$	41.8		$	42.6		$	43.1		$	11.1		$	10.2		$	11.4

	Spire									Spire Missouri									Spire Alabama
	2025			2024			2023			2025			2024			2023			2025			2024			2023
Service cost – benefits earned during    the period	$	4.2		$	4.3		$	4.8		$	3.5		$	3.5		$	4.0		$	0.7		$	0.7		$	0.7
Interest cost on accumulated    postretirement benefit obligation		7.3			8.9			8.5			5.2			6.6			6.5			2.0			2.1			1.8
Expected return on plan assets		(16.9	)		(16.0	)		(15.8	)		(11.1	)		(10.6	)		(10.5	)		(5.4	)		(5.1	)		(5.0	)
Amortization of prior service cost    (credit)		0.2			0.3			0.3			0.3			0.6			0.6			(0.1	)		(0.3	)		(0.3	)
Amortization of actuarial gain		(5.8	)		(5.8	)		(4.0	)		(5.1	)		(5.2	)		(3.2	)		(0.1	)		(0.1	)		(0.5	)
Special termination benefits		—			6.1			—			—			6.1			—			—			—			—
Subtotal		(11.0	)		(2.2	)		(6.2	)		(7.2	)		1.0			(2.6	)		(2.9	)		(2.7	)		(3.3	)
Regulatory adjustment		4.5			(3.6	)		0.6			6.2			(1.9	)		1.8			(1.8	)		(1.8	)		(1.3	)
Net postretirement benefit income	$	(6.5	)	$	(5.8	)	$	(5.6	)	$	(1.0	)	$	(0.9	)	$	(0.8	)	$	(4.7	)	$	(4.5	)	$	(4.6	)

Schedule of Amounts Recognized in Other Comprehensive Income (Loss)

	Spire									Spire Missouri									Spire Alabama
	2025			2024			2023			2025			2024			2023			2025			2024			2023
Current year actuarial (gain) loss	$	(11.9	)	$	8.8		$	14.5		$	(9.9	)	$	(0.9	)	$	5.7		$	(0.8	)	$	8.5		$	9.0
Amortization of actuarial loss		(6.5	)		(6.4	)		(6.6	)		(4.1	)		(5.9	)		(6.1	)		(2.4	)		(1.0	)		(0.9	)
Acceleration of loss recognized due to    settlement		(0.4	)		(12.9	)		(9.7	)		(0.4	)		(12.9	)		(0.6	)		—			—			(9.1	)
Current year service credit		—			0.2			—			—			0.2			—			—			—			—
Amortization of prior service credit		4.5			4.5			4.5			1.9			1.9			1.9			2.4			2.4			2.4
Subtotal		(14.3	)		(5.8	)		2.7			(12.5	)		(17.6	)		0.9			(0.8	)		9.9			1.4
Regulatory adjustment		13.0			5.2			(2.8	)		11.2			17.0			(1.0	)		0.8			(9.9	)		(1.4	)
Total recognized in OCI	$	(1.3	)	$	(0.6	)	$	(0.1	)	$	(1.3	)	$	(0.6	)	$	(0.1	)	$	—		$	—		$	—

	Spire									Spire Missouri									Spire Alabama
	2025			2024			2023			2025			2024			2023			2025			2024			2023
Current year actuarial (gain) loss	$	(36.5	)	$	(52.4	)	$	(24.4	)	$	(31.9	)	$	(40.4	)	$	(21.5	)	$	(4.0	)	$	(9.3	)	$	(1.1	)
Amortization of actuarial gain		5.8			5.8			4.0			5.1			5.2			3.2			0.1			0.1			0.5
Current year prior service cost		—			(4.7	)		—			—			(4.7	)		—			—			—			—
Amortization of prior service (cost)    credit		(0.2	)		(0.3	)		(0.3	)		(0.3	)		(0.6	)		(0.6	)		0.1			0.3			0.3
Subtotal		(30.9	)		(51.6	)		(20.7	)		(27.1	)		(40.5	)		(18.9	)		(3.8	)		(8.9	)		(0.3	)
Regulatory adjustment		30.9			51.6			20.7			27.1			40.5			18.9			3.8			8.9			0.3
Total recognized in OCI	$	—		$	—		$	—		$	—		$	—		$	—		$	—		$	—		$	—

Schedule of Accumulated and Projected Benefit Obligations

	Spire						Spire Missouri						Spire Alabama
	2025			2024			2025			2024			2025			2024
Benefit obligation, beginning of year	$	469.7		$	438.7		$	319.7		$	313.2		$	97.9		$	80.6
Service cost		17.6			15.7			11.4			10.4			5.5			4.6
Interest cost		23.5			26.7			15.8			18.9			5.1			5.1
Actuarial loss (gain)		(15.3	)		46.0			(8.5	)		25.2			(4.3	)		14.3
Plan amendments		—			0.2			—			0.2			—			—
Special termination benefits		—			2.6			—			2.6			—			—
Settlement benefits paid		(1.5	)		(37.3	)		(1.5	)		(37.3	)		—			—
Regular benefits paid		(43.8	)		(22.9	)		(29.7	)		(13.5	)		(11.2	)		(6.7	)
Benefit obligation, end of year	$	450.2		$	469.7		$	307.2		$	319.7		$	93.0		$	97.9
Accumulated benefit obligation, end of year	$	439.6		$	459.4		$	302.8		$	314.7		$	87.3		$	92.9

	Spire						Spire Missouri						Spire Alabama
	2025			2024			2025			2024			2025			2024
Benefit obligation, beginning of year	$	142.5		$	142.6		$	102.4		$	105.3		$	39.0		$	35.1
Service cost		4.2			4.3			3.5			3.5			0.7			0.7
Interest cost		7.3			8.9			5.2			6.6			2.0			2.1
Actuarial (gain) loss		(18.4	)		(1.5	)		(17.1	)		(3.2	)		(1.2	)		3.1
Plan amendments		—			(4.7	)		—			(4.7	)		—			—
Retiree drug subsidy program		—			0.2			—			0.2			—			—
Special termination benefits		—			6.1			—			6.1			—			—
Benefits paid		(4.1	)		(13.4	)		(1.9	)		(11.4	)		(2.2	)		(2.0	)
Benefit obligation, end of year	$	131.5		$	142.5		$	92.1		$	102.4		$	38.3		$	39.0

Schedule of Changes in Fair Value of Plan Assets

	Spire						Spire Missouri						Spire Alabama
	2025			2024			2025			2024			2025			2024
Fair value of plan assets, beginning of year	$	374.8		$	334.7		$	259.8		$	242.0		$	69.9		$	53.6
Actual return on plan assets		22.4			61.8			19.0			43.2			1.8			10.4
Employer contributions		75.8			38.5			37.2			25.4			30.1			12.6
Settlement benefits paid		(1.5	)		(37.3	)		(1.5	)		(37.3	)		—			—
Regular benefits paid		(43.8	)		(22.9	)		(29.7	)		(13.5	)		(11.2	)		(6.7	)
Fair value of plan assets, end of year	$	427.7		$	374.8		$	284.8		$	259.8		$	90.6		$	69.9
Funded status of plans, end of year	$	(22.5	)	$	(94.9	)	$	(22.4	)	$	(59.9	)	$	(2.4	)	$	(28.0	)

	Spire						Spire Missouri						Spire Alabama
	2025			2024			2025			2024			2025			2024
Fair value of plan assets at beginning of year	$	339.4		$	286.3		$	224.9		$	188.8		$	107.5		$	92.0
Actual return on plan assets		34.9			66.3			25.7			47.3			8.2			17.5
Employer contributions		2.7			0.2			2.7			0.2			—			—
Benefits paid		(4.1	)		(13.4	)		(1.9	)		(11.4	)		(2.2	)		(2.0	)
Fair value of plan assets, end of year	$	372.9		$	339.4		$	251.4		$	224.9		$	113.5		$	107.5
Funded status of plans, end of year	$	241.4		$	196.9		$	159.3		$	122.5		$	75.2		$	68.5

Schedule of Amounts Recognized in Balance Sheet

	Spire						Spire Missouri						Spire Alabama
	2025			2024			2025			2024			2025			2024
Noncurrent assets	$	6.2		$	—		$	3.9		$	—		$	—		$	—
Current liabilities		(0.6	)		(1.0	)		(0.6	)		(1.0	)		—			—
Noncurrent liabilities		(28.1	)		(93.9	)		(25.7	)		(58.9	)		(2.4	)		(28.0	)
Total	$	(22.5	)	$	(94.9	)	$	(22.4	)	$	(59.9	)	$	(2.4	)	$	(28.0	)

	Spire						Spire Missouri						Spire Alabama
	2025			2024			2025			2024			2025		2024
Noncurrent assets	$	288.2		$	248.3		$	206.1		$	173.9		$	75.2	$	68.5
Current liabilities		(0.4	)		(0.4	)		(0.4	)		(0.4	)		—		—
Noncurrent liabilities		(46.4	)		(51.0	)		(46.4	)		(51.0	)		—		—
Total	$	241.4		$	196.9		$	159.3		$	122.5		$	75.2	$	68.5

Schedule of Amounts in Accumulated Other Comprehensive Income (Loss) to be Recognized over Next Fiscal Year

	Spire						Spire Missouri						Spire Alabama
	2025			2024			2025			2024			2025			2024
Net actuarial loss	$	100.9		$	119.7		$	58.5		$	72.9		$	44.5		$	47.7
Prior service credit		(22.0	)		(26.4	)		(12.1	)		(14.0	)		(9.1	)		(11.5	)
Subtotal		78.9			93.3			46.4			58.9			35.4			36.2
Adjustments for amounts included in regulatory    assets		(77.8	)		(90.8	)		(45.3	)		(56.4	)		(35.4	)		(36.2	)
Total	$	1.1		$	2.5		$	1.1		$	2.5		$	—		$	—

	Spire						Spire Missouri						Spire Alabama
	2025			2024			2025			2024			2025			2024
Net actuarial gain	$	(180.6	)	$	(150.0	)	$	(154.2	)	$	(127.5	)	$	(19.3	)	$	(15.4	)
Prior service cost (credit)		2.0			2.0			2.0			2.1			—			(0.1	)
Subtotal		(178.6	)		(148.0	)		(152.2	)		(125.4	)		(19.3	)		(15.5	)
Adjustments for amounts included in regulatory    assets		178.6			148.0			152.2			125.4			19.3			15.5
Total	$	—		$	—		$	—		$	—		$	—		$	—

Defined Benefit Plan, Assumptions

	2025	2024	2023
Weighted average discount rate - Spire Missouri East plan	5.10%	6.25%	5.70%
Weighted average discount rate - Spire Missouri West plan	5.05%	6.30%	5.80%
Weighted average rate of future compensation increase	3.50%	3.00%	3.00%
Expected long-term rate of return on plan assets	6.50%	6.50%	6.50%

	2025	2024	2023
Weighted average discount rate	5.10%	6.20%/6.25%	5.65%/5.70%
Weighted average rate of future compensation increase	3.50%	3.00%	3.00%
Expected long-term rate of return on plan assets	6.75%	6.75%	6.50%

	2025	2024
Weighted average discount rate - Spire Missouri East plan	5.55%	5.10%
Weighted average discount rate - Spire Missouri West plan	5.45%	5.05%
Weighted average discount rate - Spire Alabama plans	5.55%	5.10%
Weighted average rate of future compensation increase (all plans)	3.50%	3.50%
Cash balance interest crediting rate - Spire Alabama / Spire Missouri	4.25%	4.25%

	2025	2024	2023
Weighted average discount rate – Spire Missouri plans	5.05%	6.30%	5.80%
Expected long-term rate of return on plan assets – Spire Missouri plans	5.50%	5.50%	5.50%

	2025	2024	2023
Weighted average discount rate	5.05%	6.30%	5.80%
Expected long-term rate of return on plan assets	4.75%/6.00%	4.75%/6.00%	4.75%/6.00%

Defined Benefit Plan, Plan with Projected Benefit Obligation in Excess of Plan Assets

	Spire				Spire Missouri				Spire Alabama
	2025		2024		2025		2024		2025		2024
Projected benefit obligation	$	450.2	$	469.7	$	307.2	$	319.7	$	93.0	$	97.9
Accumulated benefit obligation		439.6		459.4		302.8		314.7		87.3		92.9
Fair value of plan assets		427.7		374.8		284.8		259.8		90.6		69.9

Schedule of Allocation of Plan Assets

Spire Missouri	2025 Target			2025 Actual			2024 Target			2024 Actual
Return seeking assets		70.0	%		56.6	%		70.0	%		71.3	%
Liability hedging assets		30.0	%		40.4	%		30.0	%		24.3	%
Cash and cash equivalents		—	%		3.0	%		—	%		4.4	%
Total		100.0	%		100.0	%		100.0	%		100.0	%

Spire Alabama	2025 Target			2025 Actual			2024 Target			2024 Actual
Return seeking assets		75.0	%		72.0	%		75.0	%		74.9	%
Liability hedging assets		25.0	%		16.8	%		25.0	%		22.6	%
Cash and cash equivalents		—	%		11.2	%		—	%		2.5	%
Total		100.0	%		100.0	%		100.0	%		100.0	%

Spire Missouri	Target			2025 Actual			2024 Actual
Equity securities		60.0	%		58.6	%		60.8	%
Debt securities		40.0	%		36.0	%		38.2	%
Cash and cash equivalents		—	%		5.4	%		1.0	%
Total		100.0	%		100.0	%		100.0	%

Spire Alabama	Target			2025 Actual			2024 Actual
Equity securities		60.5	%		60.9	%		60.7	%
Debt securities		39.5	%		39.1	%		39.3	%
Total		100.0	%		100.0	%		100.0	%

Schedule of Expected Benefit Payments

	2026		2027		2028		2029		2030		2031 – 2035
Spire	$	51.2	$	46.0	$	45.3	$	42.9	$	41.7	$	196.2
Spire Missouri		38.5		33.4		32.6		30.5		29.1		133.5
Spire Alabama		8.8		8.8		8.5		8.4		8.5		42.8

	2026		2027		2028		2029		2030		2031 – 2035
Spire	$	13.4	$	13.0	$	12.5	$	12.0	$	11.6	$	54.0
Spire Missouri		10.1		9.7		9.3		8.9		8.5		38.3
Spire Alabama		3.2		3.2		3.1		3.0		3.0		15.2

Schedule of Health Care Cost Trend Rates

	2025	2024
Medical cost trend assumed for next year – Spire Alabama and Spire Missouri	7.00%	6.50%
Rate to which the medical cost trend rate is assumed to decline (the ultimate medical    cost trend rate)	5.00%	5.00%
Year the rate reaches the ultimate trend	2034	2031

Fair Value, Assets Measured on Recurring Basis

	As of September 30, 2025						As of September 30, 2024
	Quoted Prices		Significant				Quoted Prices		Significant
	in Active		Observable				in Active		Observable
	Markets		Inputs				Markets		Inputs
	(Level 1)		(Level 2)		Total		(Level 1)		(Level 2)		Total
Cash and cash equivalents	$	27.4	$	—	$	27.4	$	12.7	$	—	$	12.7
Equity funds - global (including U.S.)		—		181.5		181.5		—		152.4		152.4
Real asset funds		—		61.4		61.4		—		57.7		57.7
Debt securities:
U.S. bond funds		50.7		—		50.7		55.6		—		55.6
U.S. government index funds		38.3		—		38.3		33.9		—		33.9
Global funds (including U.S.)		—		68.4		68.4		—		62.5		62.5
Total	$	116.4	$	311.3	$	427.7	$	102.2	$	272.6	$	374.8

	As of September 30, 2025						As of September 30, 2024
	Quoted Prices		Significant				Quoted Prices		Significant
	in Active		Observable				in Active		Observable
	Markets		Inputs				Markets		Inputs
	(Level 1)		(Level 2)		Total		(Level 1)		(Level 2)		Total
Cash and cash equivalents	$	13.1	$	—	$	13.1	$	1.6	$	—	$	1.6
U.S. stock/bond mutual funds		294.5		44.4		338.9		229.1		89.0		318.1
International fund		1.4		19.5		20.9		—		19.7		19.7
Total	$	309.0	$	63.9	$	372.9	$	230.7	$	108.7	$	339.4

	As of September 30, 2025						As of September 30, 2024
	Quoted Prices		Significant				Quoted Prices		Significant
	in Active		Observable				in Active		Observable
	Markets		Inputs				Markets		Inputs
	(Level 1)		(Level 2)		Total		(Level 1)		(Level 2)		Total
Cash and cash equivalents	$	10.1	$	—	$	10.1	$	9.9	$	—	$	9.9
Equity funds - global (including U.S.)		—		117.1		117.1		—		102.2		102.2
Real asset funds		—		43.1		43.1		—		40.2		40.2
Debt securities:
U.S. bond funds		35.1		—		35.1		38.2		—		38.2
U.S. government index funds		30.2		—		30.2		25.2		—		25.2
Global funds (including U.S.)		—		49.2		49.2		—		44.1		44.1
Total	$	75.4	$	209.4	$	284.8	$	73.3	$	186.5	$	259.8

	As of September 30, 2025						As of September 30, 2024
	Quoted Prices		Significant				Quoted Prices		Significant
	in Active		Observable				in Active		Observable
	Markets		Inputs				Markets		Inputs
	(Level 1)		(Level 2)		Total		(Level 1)		(Level 2)		Total
Cash and cash equivalents	$	12.1	$	—	$	12.1	$	0.7	$	—	$	0.7
U.S. stock/bond mutual funds		239.3		—		239.3		224.2		—	$	224.2
Total	$	251.4	$	—	$	251.4	$	224.9	$	—	$	224.9

	As of September 30, 2025						As of September 30, 2024
	Quoted Prices		Significant				Quoted Prices		Significant
	in Active		Observable				in Active		Observable
	Markets		Inputs				Markets		Inputs
	(Level 1)		(Level 2)		Total		(Level 1)		(Level 2)		Total
Cash and cash equivalents	$	10.2	$	—	$	10.2	$	1.7	$	—	$	1.7
Equity funds - global (including U.S.)		—		41.3		41.3		—		30.6		30.6
Real asset funds		—		11.7		11.7		—		10.6		10.6
Debt securities:
U.S. bond funds		9.9		—		9.9		10.6		—		10.6
U.S. government index funds		5.2		—		5.2		5.2		—		5.2
Global funds (including U.S.)		—		12.3		12.3		—		11.2		11.2
Total	$	25.3	$	65.3	$	90.6	$	17.5	$	52.4	$	69.9

	As of September 30, 2025						As of September 30, 2024
	Quoted Prices		Significant				Quoted Prices		Significant
	in Active		Observable				in Active		Observable
	Markets		Inputs				Markets		Inputs
	(Level 1)		(Level 2)		Total		(Level 1)		(Level 2)		Total
U.S. stock/bond mutual funds	$	49.6	$	44.4	$	94.0	$	—	$	89.0	$	89.0
International fund		—		19.5		19.5		—		18.5		18.5
Total	$	49.6	$	63.9	$	113.5	$	—	$	107.5	$	107.5