Note 13 - Pension Plans and Other Postretirement Benefits - Reconciliation of Pension Benefit Obligation (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Special termination benefits	$ 6.1	$ 0.0	$ 0.0
Spire Missouri [Member]
Special termination benefits	6.1	0.0	0.0
Spire Alabama Inc [Member]
Special termination benefits	0.0	0.0	0.0
Pension Plan [Member]
Benefit obligation, beginning of year	438.7	464.4
Service cost – benefits earned during the period	15.7	16.3	20.0
Interest cost on projected benefit obligation	26.7	25.1	21.5
Actuarial loss (gain)	46.0	(1.1)
Plan amendments	0.2	0.0
Settlement loss	0.0	9.7
Special termination benefits	2.6	0.0
Settlement benefits paid	(37.3)	(48.8)
Regular benefits paid	(22.9)	(26.9)
Benefit obligation, end of year	469.7	438.7	464.4
Accumulated benefit obligation, end of year	459.4	433.1
Plan amendments	(0.2)	(0.0)
Special termination benefits	2.6	0.0	0.0
Pension Plan [Member] | Spire Missouri [Member]
Benefit obligation, beginning of year	313.2	320.6
Service cost – benefits earned during the period	10.4	11.4	14.4
Interest cost on projected benefit obligation	18.9	17.8	14.7
Actuarial loss (gain)	25.2	(4.9)
Plan amendments	0.2	0.0
Settlement loss	0.0	0.6
Special termination benefits	2.6	0.0
Settlement benefits paid	(37.3)	(9.9)
Regular benefits paid	(13.5)	(22.4)
Benefit obligation, end of year	319.7	313.2	320.6
Accumulated benefit obligation, end of year	314.7	309.5
Plan amendments	(0.2)	(0.0)
Special termination benefits	2.6	0.0	0.0
Pension Plan [Member] | Spire Alabama Inc [Member]
Benefit obligation, beginning of year	80.6	97.8
Service cost – benefits earned during the period	4.6	4.2	4.8
Interest cost on projected benefit obligation	5.1	4.8	4.7
Actuarial loss (gain)	14.3	5.4
Plan amendments	0.0	0.0
Settlement loss	0.0	9.1
Special termination benefits	0.0	0.0
Settlement benefits paid	0.0	(38.8)
Regular benefits paid	(6.7)	(1.9)
Benefit obligation, end of year	97.9	80.6	97.8
Accumulated benefit obligation, end of year	92.9	78.8
Plan amendments	(0.0)	(0.0)
Special termination benefits	0.0	0.0	0.0
Other Postretirement Benefits Plan [Member]
Benefit obligation, beginning of year	142.6	148.1
Service cost – benefits earned during the period	4.3	4.8	7.5
Interest cost on projected benefit obligation	8.9	8.5	6.0
Actuarial loss (gain)	(1.5)	(10.6)
Plan amendments	4.7	0.0
Regular benefits paid	(13.4)	(8.4)
Benefit obligation, end of year	142.5	142.6	148.1
Plan amendments	(4.7)	0.0
Retiree drug subsidy program	0.2	0.2
Special termination benefits	6.1	0.0
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member]
Benefit obligation, beginning of year	105.3	114.8
Service cost – benefits earned during the period	3.5	4.0	6.3
Interest cost on projected benefit obligation	6.6	6.5	4.5
Actuarial loss (gain)	(3.2)	(13.4)
Plan amendments	4.7	0.0
Regular benefits paid	(11.4)	(6.8)
Benefit obligation, end of year	102.4	105.3	114.8
Plan amendments	(4.7)	0.0
Retiree drug subsidy program	0.2	0.2
Special termination benefits	6.1	0.0
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member]
Benefit obligation, beginning of year	35.1	29.9
Service cost – benefits earned during the period	0.7	0.7	1.1
Interest cost on projected benefit obligation	2.1	1.8	1.4
Actuarial loss (gain)	3.1	4.2
Plan amendments	0.0	0.0
Regular benefits paid	(2.0)	(1.5)
Benefit obligation, end of year	39.0	35.1	$ 29.9
Plan amendments	0.0	0.0
Retiree drug subsidy program	0.0	0.0
Special termination benefits	$ 0.0	$ 0.0