Pension Plans and Other Postretirement Benefits - Other Changes in Plan Assets and Pension Benefit Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Current year actuarial (gain) loss	$ (11.9)	$ 8.8	$ 14.5
Amortization of actuarial loss	(6.5)	(6.4)	(6.6)
Acceleration of loss recognized due to settlement	(0.4)	(12.9)	(9.7)
Current year service credit	0.0	0.2	0.0
Amortization of prior service credit	4.5	4.5	4.5
Subtotal	(14.3)	(5.8)	2.7
Regulatory adjustment	13.0	5.2	(2.8)
Total recognized in OCI	(1.3)	(0.6)	(0.1)
Pension Plan [Member] | Spire Missouri [Member]
Defined Benefit Plan Disclosure [Line Items]
Current year actuarial (gain) loss	(9.9)	(0.9)	5.7
Amortization of actuarial loss	(4.1)	(5.9)	(6.1)
Acceleration of loss recognized due to settlement	(0.4)	(12.9)	(0.6)
Current year service credit	0.0	0.2	0.0
Amortization of prior service credit	1.9	1.9	1.9
Subtotal	(12.5)	(17.6)	0.9
Regulatory adjustment	11.2	17.0	(1.0)
Total recognized in OCI	(1.3)	(0.6)	(0.1)
Pension Plan [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Disclosure [Line Items]
Current year actuarial (gain) loss	(0.8)	8.5	9.0
Amortization of actuarial loss	(2.4)	(1.0)	(0.9)
Acceleration of loss recognized due to settlement	0.0	0.0	(9.1)
Current year service credit	0.0	0.0	0.0
Amortization of prior service credit	2.4	2.4	2.4
Subtotal	(0.8)	9.9	1.4
Regulatory adjustment	0.8	(9.9)	(1.4)
Total recognized in OCI	0.0	0.0	0.0
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Current year actuarial (gain) loss	(36.5)	(52.4)	(24.4)
Amortization of actuarial loss	5.8	5.8	4.0
Amortization of prior service credit	(0.2)	(0.3)	(0.3)
Subtotal	(30.9)	(51.6)	(20.7)
Regulatory adjustment	30.9	51.6	20.7
Total recognized in OCI	0.0	0.0	0.0
Current year prior service cost	0.0	(4.7)	0.0
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member]
Defined Benefit Plan Disclosure [Line Items]
Current year actuarial (gain) loss	(31.9)	(40.4)	(21.5)
Amortization of actuarial loss	5.1	5.2	3.2
Amortization of prior service credit	(0.3)	(0.6)	(0.6)
Subtotal	(27.1)	(40.5)	(18.9)
Regulatory adjustment	27.1	40.5	18.9
Total recognized in OCI	0.0	0.0	0.0
Current year prior service cost		(4.7)	0.0
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Disclosure [Line Items]
Current year actuarial (gain) loss	(4.0)	(9.3)	(1.1)
Amortization of actuarial loss	0.1	0.1	0.5
Amortization of prior service credit	0.1	0.3	0.3
Subtotal	(3.8)	(8.9)	(0.3)
Regulatory adjustment	3.8	8.9	0.3
Total recognized in OCI	$ 0.0	0.0	0.0
Current year prior service cost		$ 0.0	$ 0.0