Note 13 - Pension Plans and Other Postretirement Benefits - Reconciliation of Fair Value of Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Spire Missouri [Member]
Fair value of plan assets, beginning of year	$ 235.9	$ 364.0
Actual return on plan assets		(66.7)
Employer contributions		37.8
Settlement benefits paid		(87.0)
Regular benefits paid		(12.2)
Fair value of plan assets, end of year		235.9
Funded status of plans, end of year		(84.7)
Spire Alabama Inc [Member]
Fair value of plan assets, beginning of year	57.3	82.8
Actual return on plan assets		(15.6)
Employer contributions		14.4
Settlement benefits paid		(22.3)
Regular benefits paid		(2.0)
Fair value of plan assets, end of year		57.3
Funded status of plans, end of year		(40.5)
Pension Plan [Member]
Fair value of plan assets, beginning of year	332.7	498.9
Actual return on plan assets	9.1	(92.5)
Employer contributions	58.9	52.4
Settlement benefits paid	(39.1)	(109.3)
Regular benefits paid	(26.9)	(16.8)
Fair value of plan assets, end of year	334.7	332.7
Funded status of plans, end of year	(104.0)	(131.7)
Pension Plan [Member] | Spire Missouri [Member]
Fair value of plan assets, beginning of year	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	235.9
Funded status of plans, end of year	(71.2)
Pension Plan [Member] | Spire Alabama Inc [Member]
Fair value of plan assets, beginning of year	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	57.3
Funded status of plans, end of year	(27.0)
Other Postretirement Benefits Plan [Member]
Fair value of plan assets, beginning of year	264.7	326.9
Actual return on plan assets	29.9	(51.6)
Employer contributions	0.1	0.4
Regular benefits paid	(8.4)	(11.0)
Fair value of plan assets, end of year	286.3	264.7
Funded status of plans, end of year	143.7	116.6
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member]
Fair value of plan assets, beginning of year	176.7	221.7
Actual return on plan assets	18.8	(36.2)
Employer contributions	0.1	0.4
Regular benefits paid	(6.8)	(9.2)
Fair value of plan assets, end of year	188.8	176.7
Funded status of plans, end of year	83.5	61.9
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member]
Fair value of plan assets, beginning of year	83.3	99.4
Actual return on plan assets	10.2	(14.3)
Employer contributions	0.0	0.0
Regular benefits paid	(1.5)	(1.8)
Fair value of plan assets, end of year	92.0	83.3
Funded status of plans, end of year	$ 56.9	$ 53.4