Note 13 - Pension Plans and Other Postretirement Benefits - Reconciliation of Fair Value of Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023
Spire Missouri [Member]
Fair value of plan assets, beginning of year	$ 242.0	$ 235.9
Actual return on plan assets		7.5
Employer contributions		30.3
Settlement benefits paid		(9.3)
Regular benefits paid		(22.4)
Fair value of plan assets, end of year		242.0
Funded status of plans, end of year		(71.2)
Spire Alabama Inc [Member]
Fair value of plan assets, beginning of year	53.6	57.3
Actual return on plan assets		0.0
Employer contributions		27.9
Settlement benefits paid		(29.7)
Regular benefits paid		(1.9)
Fair value of plan assets, end of year		53.6
Funded status of plans, end of year		(27.0)
Pension Plan [Member]
Fair value of plan assets, beginning of year	334.7	332.7
Actual return on plan assets	61.8	9.1
Employer contributions	38.5	58.9
Settlement benefits paid	(37.3)	(39.1)
Regular benefits paid	(22.9)	(26.9)
Fair value of plan assets, end of year	374.8	334.7
Funded status of plans, end of year	(94.9)	(104.0)
Pension Plan [Member] | Spire Missouri [Member]
Fair value of plan assets, beginning of year	242.0
Actual return on plan assets	43.2
Employer contributions	25.4
Settlement benefits paid	(37.3)
Regular benefits paid	(13.5)
Fair value of plan assets, end of year	259.8	242.0
Funded status of plans, end of year	(59.9)
Pension Plan [Member] | Spire Alabama Inc [Member]
Fair value of plan assets, beginning of year	53.6
Actual return on plan assets	10.4
Employer contributions	12.6
Settlement benefits paid	0.0
Regular benefits paid	(6.7)
Fair value of plan assets, end of year	69.9	53.6
Funded status of plans, end of year	(28.0)
Other Postretirement Benefits Plan [Member]
Fair value of plan assets, beginning of year	286.3	264.7
Actual return on plan assets	66.3	29.9
Employer contributions	0.2	0.1
Regular benefits paid	(13.4)	(8.4)
Fair value of plan assets, end of year	339.4	286.3
Funded status of plans, end of year	196.9	143.7
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member]
Fair value of plan assets, beginning of year	188.8	176.7
Actual return on plan assets	47.3	18.8
Employer contributions	0.2	0.1
Regular benefits paid	(11.4)	(6.8)
Fair value of plan assets, end of year	224.9	188.8
Funded status of plans, end of year	122.5	83.5
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member]
Fair value of plan assets, beginning of year	92.0	83.3
Actual return on plan assets	17.5	10.2
Employer contributions	0.0	0.0
Regular benefits paid	(2.0)	(1.5)
Fair value of plan assets, end of year	107.5	92.0
Funded status of plans, end of year	$ 68.5	$ 56.9