Pension Plans and Other Postretirement Benefits - Reconciliation of Fair Value of Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024
Pension Plan [Member]
Fair value of plan assets, beginning of year	$ 374.8	$ 334.7
Actual return on plan assets	22.4	61.8
Employer contributions	75.8	38.5
Settlement benefits paid	(1.5)	(37.3)
Regular benefits paid	(43.8)	(22.9)
Fair value of plan assets, end of year	427.7	374.8
Funded status of plans, end of year	(22.5)	(94.9)
Pension Plan [Member] | Spire Missouri [Member]
Fair value of plan assets, beginning of year	259.8	242.0
Actual return on plan assets	19.0	43.2
Employer contributions	37.2	25.4
Settlement benefits paid	(1.5)	(37.3)
Regular benefits paid	(29.7)	(13.5)
Fair value of plan assets, end of year	284.8	259.8
Funded status of plans, end of year	(22.4)	(59.9)
Pension Plan [Member] | Spire Alabama Inc [Member]
Fair value of plan assets, beginning of year	69.9	53.6
Actual return on plan assets	1.8	10.4
Employer contributions	30.1	12.6
Settlement benefits paid	0.0	0.0
Regular benefits paid	(11.2)	(6.7)
Fair value of plan assets, end of year	90.6	69.9
Funded status of plans, end of year	(2.4)	(28.0)
Other Postretirement Benefits Plan [Member]
Fair value of plan assets, beginning of year	339.4	286.3
Actual return on plan assets	34.9	66.3
Employer contributions	2.7	0.2
Regular benefits paid	(4.1)	(13.4)
Fair value of plan assets, end of year	372.9	339.4
Funded status of plans, end of year	241.4	196.9
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member]
Fair value of plan assets, beginning of year	224.9	188.8
Actual return on plan assets	25.7	47.3
Employer contributions	2.7	0.2
Regular benefits paid	(1.9)	(11.4)
Fair value of plan assets, end of year	251.4	224.9
Funded status of plans, end of year	159.3	122.5
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member]
Fair value of plan assets, beginning of year	107.5	92.0
Actual return on plan assets	8.2	17.5
Regular benefits paid	(2.2)	(2.0)
Fair value of plan assets, end of year	113.5	107.5
Funded status of plans, end of year	$ 75.2	$ 68.5