Pension Plans and Other Postretirement Benefits - Net Pension Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Spire Missouri [Member]
Defined Benefit Plan Disclosure [Line Items]
Special termination benefits		$ 6.1
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the period	$ 17.6	15.7	$ 16.3
Interest cost on projected benefit obligation	23.5	26.7	25.1
Expected return on plan assets	(25.9)	(24.8)	(24.5)
Amortization of prior service credit	(4.5)	(4.5)	(4.5)
Amortization of actuarial loss	6.5	6.4	6.6
Loss on lump-sum settlements and curtailments	0.4	12.9	9.7
Subtotal	17.6	35.0	28.7
Regulatory adjustment	36.3	18.4	26.3
Net pension cost	53.9	53.4	55.0
Special termination benefits	0.0	2.6	0.0
Pension Plan [Member] | Spire Missouri [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the period	11.4	10.4	11.4
Interest cost on projected benefit obligation	15.8	18.9	17.8
Expected return on plan assets	(17.6)	(17.0)	(18.0)
Amortization of prior service credit	(1.9)	(1.9)	(1.9)
Amortization of actuarial loss	4.1	5.9	6.1
Loss on lump-sum settlements and curtailments	0.4	12.9	0.6
Subtotal	12.2	31.8	16.0
Regulatory adjustment	29.6	10.8	27.1
Net pension cost	41.8	42.6	43.1
Special termination benefits	0.0	2.6	0.0
Pension Plan [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the period	5.5	4.6	4.2
Interest cost on projected benefit obligation	5.1	5.1	4.8
Expected return on plan assets	(5.3)	(4.8)	(3.5)
Amortization of prior service credit	(2.4)	(2.4)	(2.4)
Amortization of actuarial loss	2.4	1.0	0.9
Loss on lump-sum settlements and curtailments	0.0	0.0	9.1
Subtotal	5.3	3.5	13.1
Regulatory adjustment	5.8	6.7	(1.7)
Net pension cost	11.1	10.2	11.4
Special termination benefits	0.0	0.0	0.0
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the period	4.2	4.3	4.8
Interest cost on projected benefit obligation	7.3	8.9	8.5
Expected return on plan assets	(16.9)	(16.0)	(15.8)
Amortization of prior service credit	0.2	0.3	0.3
Amortization of actuarial loss	(5.8)	(5.8)	(4.0)
Subtotal	(11.0)	(2.2)	(6.2)
Regulatory adjustment	4.5	(3.6)	0.6
Net pension cost	(6.5)	(5.8)	(5.6)
Special termination benefits	0.0	6.1	0.0
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the period	3.5	3.5	4.0
Interest cost on projected benefit obligation	5.2	6.6	6.5
Expected return on plan assets	(11.1)	(10.6)	(10.5)
Amortization of prior service credit	0.3	0.6	0.6
Amortization of actuarial loss	(5.1)	(5.2)	(3.2)
Subtotal	(7.2)	1.0	(2.6)
Regulatory adjustment	6.2	(1.9)	1.8
Net pension cost	(1.0)	(0.9)	(0.8)
Special termination benefits	0.0	6.1	0.0
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the period	0.7	0.7	0.7
Interest cost on projected benefit obligation	2.0	2.1	1.8
Expected return on plan assets	(5.4)	(5.1)	(5.0)
Amortization of prior service credit	(0.1)	(0.3)	(0.3)
Amortization of actuarial loss	(0.1)	(0.1)	(0.5)
Subtotal	(2.9)	(2.7)	(3.3)
Regulatory adjustment	(1.8)	(1.8)	(1.3)
Net pension cost	$ (4.7)	(4.5)	(4.6)
Special termination benefits		$ 0.0	$ 0.0