Pension Plans and Other Postretirement Benefits - Reconciliation of Pension Benefit Obligation (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Spire Missouri [Member]
Special termination benefits		$ 6.1
Pension Plan [Member]
Benefit obligation, beginning of year	$ 469.7	438.7
Service cost – benefits earned during the period	17.6	15.7	$ 16.3
Interest cost on projected benefit obligation	23.5	26.7	25.1
Actuarial loss (gain)	(15.3)	46.0
Plan amendments	0.0	0.2
Special termination benefits	0.0	2.6
Settlement benefits paid	(1.5)	(37.3)
Regular benefits paid	(43.8)	(22.9)
Benefit obligation, end of year	450.2	469.7	438.7
Accumulated benefit obligation, end of year	439.6	459.4
Special termination benefits	0.0	2.6	0.0
Pension Plan [Member] | Spire Missouri [Member]
Benefit obligation, beginning of year	319.7	313.2
Service cost – benefits earned during the period	11.4	10.4	11.4
Interest cost on projected benefit obligation	15.8	18.9	17.8
Actuarial loss (gain)	(8.5)	25.2
Plan amendments	0.0	0.2
Special termination benefits	0.0	2.6
Settlement benefits paid	(1.5)	(37.3)
Regular benefits paid	(29.7)	(13.5)
Benefit obligation, end of year	307.2	319.7	313.2
Accumulated benefit obligation, end of year	302.8	314.7
Special termination benefits	0.0	2.6	0.0
Pension Plan [Member] | Spire Alabama Inc [Member]
Benefit obligation, beginning of year	97.9	80.6
Service cost – benefits earned during the period	5.5	4.6	4.2
Interest cost on projected benefit obligation	5.1	5.1	4.8
Actuarial loss (gain)	(4.3)	14.3
Plan amendments	0.0	0.0
Special termination benefits	0.0	0.0
Settlement benefits paid	0.0	0.0
Regular benefits paid	(11.2)	(6.7)
Benefit obligation, end of year	93.0	97.9	80.6
Accumulated benefit obligation, end of year	87.3	92.9
Special termination benefits	0.0	0.0	0.0
Other Postretirement Benefits Plan [Member]
Benefit obligation, beginning of year	142.5	142.6
Service cost – benefits earned during the period	4.2	4.3	4.8
Interest cost on projected benefit obligation	7.3	8.9	8.5
Actuarial loss (gain)	(18.4)	(1.5)
Plan amendments	0.0	(4.7)
Regular benefits paid	(4.1)	(13.4)
Benefit obligation, end of year	131.5	142.5	142.6
Retiree drug subsidy program	0.0	0.2
Special termination benefits	0.0	6.1	0.0
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member]
Benefit obligation, beginning of year	102.4	105.3
Service cost – benefits earned during the period	3.5	3.5	4.0
Interest cost on projected benefit obligation	5.2	6.6	6.5
Actuarial loss (gain)	(17.1)	(3.2)
Plan amendments	0.0	(4.7)
Regular benefits paid	(1.9)	(11.4)
Benefit obligation, end of year	92.1	102.4	105.3
Retiree drug subsidy program	0.0	0.2
Special termination benefits	0.0	6.1	0.0
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member]
Benefit obligation, beginning of year	39.0	35.1
Service cost – benefits earned during the period	0.7	0.7	0.7
Interest cost on projected benefit obligation	2.0	2.1	1.8
Actuarial loss (gain)	(1.2)	3.1
Regular benefits paid	(2.2)	(2.0)
Benefit obligation, end of year	$ 38.3	39.0	35.1
Special termination benefits		$ 0.0	$ 0.0