Note 13 - Pension Plans and Other Postretirement Benefits - Other Changes in Plan Assets and Pension Benefit Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Pension Plan [Member]
Current year actuarial loss (gain)	$ 8.8	$ 14.5	$ (16.9)
Amortization of actuarial loss	(6.4)	(6.6)	(11.9)
Acceleration of loss recognized due to settlement	(12.9)	(9.7)	(33.6)
Current year service credit	0.2	0.0	0.0
Amortization of prior service credit	4.5	4.5	4.5
Subtotal	(5.8)	2.7	(57.9)
Regulatory adjustment	5.2	(2.8)	56.3
Total recognized in OCI	(0.6)	(0.1)	(1.6)
Pension Plan [Member] | Spire Missouri [Member]
Current year actuarial loss (gain)	(0.9)	5.7	0.9
Amortization of actuarial loss	(5.9)	(6.1)	(9.6)
Acceleration of loss recognized due to settlement	(12.9)	(0.6)	(27.3)
Current year service credit	0.2	0.0	0.0
Amortization of prior service credit	1.9	1.9	1.9
Subtotal	(17.6)	0.9	(34.1)
Regulatory adjustment	17.0	(1.0)	32.5
Total recognized in OCI	(0.6)	(0.1)	(1.6)
Pension Plan [Member] | Spire Alabama Inc [Member]
Current year actuarial loss (gain)	8.5	9.0	(16.0)
Amortization of actuarial loss	(1.0)	(0.9)	(2.4)
Acceleration of loss recognized due to settlement	0.0	(9.1)	(6.3)
Current year service credit	0.0	0.0	0.0
Amortization of prior service credit	2.4	2.4	2.4
Subtotal	9.9	1.4	(22.3)
Regulatory adjustment	(9.9)	(1.4)	22.3
Total recognized in OCI	0.0	0.0	0.0
Other Postretirement Benefits Plan [Member]
Current year actuarial loss (gain)	(52.4)	(24.4)	15.8
Amortization of actuarial loss	5.8	4.0	2.2
Amortization of prior service credit	(0.3)	(0.3)	(1.0)
Subtotal	(51.6)	(20.7)	10.7
Regulatory adjustment	51.6	20.7	(10.7)
Total recognized in OCI	0.0	0.0	0.0
Current year prior service cost	(4.7)	0.0	(6.3)
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member]
Current year actuarial loss (gain)	(40.4)	(21.5)	9.8
Amortization of actuarial loss	5.2	3.2	1.9
Amortization of prior service credit	(0.6)	(0.6)	(0.7)
Subtotal	(40.5)	(18.9)	9.9
Regulatory adjustment	40.5	18.9	(9.9)
Total recognized in OCI	0.0	0.0	0.0
Current year prior service cost	(4.7)	0.0	(1.1)
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member]
Current year actuarial loss (gain)	(9.3)	(1.1)	5.9
Amortization of actuarial loss	0.1	0.5	0.0
Amortization of prior service credit	0.3	0.3	(0.3)
Subtotal	(8.9)	(0.3)	0.4
Regulatory adjustment	8.9	0.3	(0.4)
Total recognized in OCI	0.0	0.0	0.0
Current year prior service cost	$ 0.0	$ 0.0	$ (5.2)