Note 13 - Pension Plans and Other Postretirement Benefits - Net Pension Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Pension Plan [Member]
Service cost – benefits earned during the period	$ 16.3	$ 20.0	$ 21.7
Interest cost on projected benefit obligation	25.1	21.5	20.7
Expected return on plan assets	(24.5)	(31.3)	(31.6)
Amortization of prior service credit	(4.5)	(4.5)	(3.1)
Amortization of actuarial loss	6.6	11.9	14.9
Loss on lump-sum settlements and curtailments	9.7	33.6	18.2
Subtotal	28.7	51.2	40.8
Regulatory adjustment	26.3	12.5	20.6
Net pension cost	55.0	63.7	61.4
Pension Plan [Member] | Spire Missouri [Member]
Service cost – benefits earned during the period	11.4	14.4	15.4
Interest cost on projected benefit obligation	17.8	14.7	14.2
Expected return on plan assets	(18.0)	(22.7)	(22.5)
Amortization of prior service credit	(1.9)	(1.9)	(0.6)
Amortization of actuarial loss	6.1	9.6	11.0
Loss on lump-sum settlements and curtailments	0.6	27.3	11.6
Subtotal	16.0	41.4	29.1
Regulatory adjustment	27.1	9.9	19.0
Net pension cost	43.1	51.3	48.1
Pension Plan [Member] | Spire Alabama Inc [Member]
Service cost – benefits earned during the period	4.2	4.8	5.5
Interest cost on projected benefit obligation	4.8	4.7	4.6
Expected return on plan assets	(3.5)	(5.2)	(5.8)
Amortization of prior service credit	(2.4)	(2.4)	(2.3)
Amortization of actuarial loss	0.9	2.4	3.9
Loss on lump-sum settlements and curtailments	9.1	6.3	6.6
Subtotal	13.1	10.6	12.5
Regulatory adjustment	(1.7)	1.7	0.7
Net pension cost	11.4	12.3	13.2
Other Postretirement Benefits Plan [Member]
Service cost – benefits earned during the period	4.8	7.5	7.3
Interest cost on projected benefit obligation	8.5	6.0	6.0
Expected return on plan assets	(15.8)	(16.8)	(16.1)
Amortization of prior service credit	0.3	1.0	1.0
Amortization of actuarial loss	(4.0)	(2.2)	(1.6)
Subtotal	(6.2)	(4.5)	(3.4)
Regulatory adjustment	0.6	3.3	13.2
Net pension cost	(5.6)	(1.2)	9.8
Other Postretirement Benefits Plan [Member] | Spire Missouri [Member]
Service cost – benefits earned during the period	4.0	6.3	6.2
Interest cost on projected benefit obligation	6.5	4.5	4.5
Expected return on plan assets	(10.5)	(11.3)	(10.9)
Amortization of prior service credit	0.6	0.7	0.7
Amortization of actuarial loss	(3.2)	(1.9)	(1.5)
Subtotal	(2.6)	(1.7)	(1.0)
Regulatory adjustment	1.8	5.0	15.0
Net pension cost	(0.8)	3.3	14.0
Other Postretirement Benefits Plan [Member] | Spire Alabama Inc [Member]
Service cost – benefits earned during the period	0.7	1.1	0.9
Interest cost on projected benefit obligation	1.8	1.4	1.3
Expected return on plan assets	(5.0)	(5.2)	(4.9)
Amortization of prior service credit	(0.3)	0.3	0.3
Amortization of actuarial loss	(0.5)	0.0	0.0
Subtotal	(3.3)	(2.4)	(2.4)
Regulatory adjustment	(1.3)	(1.8)	(1.8)
Net pension cost	$ (4.6)	$ (4.2)	$ (4.2)