Note 9 - Pension Plans and Other Postretirement Benefits - Net Pension Costs (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Pension Plan [Member]
Service cost – benefits earned during the period	$ 3.9	$ 3.9	$ 7.9	$ 8.0
Interest cost on projected benefit obligation	6.6	5.9	13.4	12.3
Expected return on plan assets	(6.1)	(5.7)	(12.4)	(12.1)
Amortization of prior service credit	(1.2)	(1.2)	(2.3)	(2.3)
Amortization of actuarial loss	1.6	1.6	3.2	3.2
Loss on lump-sum settlements	0.0	0.0	0.0	8.6
Subtotal	4.8	4.5	9.8	17.7
Regulatory adjustment	8.8	10.4	17.5	12.2
Net pension cost	13.6	14.9	27.3	29.9
Pension Plan [Member] | Spire Missouri [Member]
Service cost – benefits earned during the period	2.6	2.9	5.2	5.7
Interest cost on projected benefit obligation	4.7	4.4	9.5	8.9
Expected return on plan assets	(4.2)	(4.5)	(8.5)	(9.0)
Amortization of prior service credit	(0.5)	(0.5)	(1.0)	(1.0)
Amortization of actuarial loss	1.4	1.6	2.9	3.1
Subtotal	4.0	3.9	8.1	7.7
Regulatory adjustment	6.3	8.1	12.6	16.2
Net pension cost	10.3	12.0	20.7	23.9
Pension Plan [Member] | Spire Alabama Inc [Member]
Service cost – benefits earned during the period	1.1	0.8	2.3	1.9
Interest cost on projected benefit obligation	1.2	0.8	2.5	2.1
Expected return on plan assets	(1.2)	(0.5)	(2.4)	(1.6)
Amortization of prior service credit	(0.6)	(0.6)	(1.2)	(1.2)
Amortization of actuarial loss	0.3	0.2	0.5	0.3
Loss on lump-sum settlements	0.0	0.0	0.0	8.6
Subtotal	0.8	0.7	1.7	10.1
Regulatory adjustment	2.3	2.1	4.5	(4.4)
Net pension cost	3.1	2.8	6.2	5.7
Other Postretirement Benefits Plan [Member]
Service cost – benefits earned during the period	1.0	1.2	2.1	2.4
Interest cost on projected benefit obligation	2.4	2.2	4.5	4.3
Expected return on plan assets	(4.2)	(3.9)	(8.2)	(7.8)
Amortization of prior service credit	0.1	0.0	0.2	0.1
Amortization of actuarial loss	(1.1)	(1.1)	(2.1)	(2.1)
Subtotal	(1.8)	(1.6)	(3.5)	(3.1)
Regulatory adjustment	0.3	0.2	0.5	0.3
Net pension cost	(1.5)	(1.4)	(3.0)	(2.8)
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member]
Service cost – benefits earned during the period	0.9	1.0	1.8	2.0
Interest cost on projected benefit obligation	1.7	1.7	3.3	3.3
Expected return on plan assets	(2.7)	(2.7)	(5.4)	(5.3)
Amortization of prior service credit	0.1	0.1	0.3	0.3
Amortization of actuarial loss	(0.9)	(0.8)	(1.8)	(1.6)
Subtotal	(0.9)	(0.7)	(1.8)	(1.3)
Regulatory adjustment	0.7	0.7	1.3	1.3
Net pension cost	(0.2)	0.0	(0.5)	0.0
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member]
Service cost – benefits earned during the period	0.1	0.1	0.3	0.3
Interest cost on projected benefit obligation	0.6	0.5	1.1	0.9
Expected return on plan assets	(1.3)	(1.3)	(2.6)	(2.5)
Amortization of prior service credit	0.0	(0.1)	(0.1)	(0.2)
Amortization of actuarial loss	(0.1)	(0.2)	(0.1)	(0.3)
Subtotal	(0.7)	(1.0)	(1.4)	(1.8)
Regulatory adjustment	(0.5)	(0.4)	(0.9)	(0.9)
Net pension cost	$ (1.2)	$ (1.4)	$ (2.3)	$ (2.7)