Pension Plans and Other Postretirement Benefits - Schedule of Net Periodic Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	$ 21.7	$ 22.5	$ 19.3
Interest cost on projected benefit obligation	20.7	22.6	28.2
Expected return on plan assets	(31.6)	(35.0)	(36.3)
Amortization of prior service (credit) cost	(3.1)	(2.5)	(1.1)
Amortization of actuarial (gain) loss	14.9	14.4	9.3
Loss on lump-sum settlements and curtailments	18.2	31.6	0.0
Subtotal	40.8	53.6	19.4
Regulatory adjustment	20.6	6.6	39.6
Net pension cost	61.4	60.2	59.0
Other Postretirement Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	7.3	5.9	7.4
Interest cost on projected benefit obligation	6.0	6.3	9.0
Expected return on plan assets	(16.1)	(16.7)	(16.2)
Amortization of prior service (credit) cost	1.0	(0.5)	(0.1)
Amortization of actuarial (gain) loss	(1.6)	(2.0)	(0.5)
Subtotal	(3.4)	(7.0)	(0.4)
Regulatory adjustment	13.2	16.0	10.0
Net pension cost	9.8	9.0	9.6
Spire Missouri | Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	15.4	15.7	12.4
Interest cost on projected benefit obligation	14.2	15.8	19.8
Expected return on plan assets	(22.5)	(24.6)	(25.5)
Amortization of prior service (credit) cost	(0.6)	0.1	0.9
Amortization of actuarial (gain) loss	11.0	11.3	8.7
Loss on lump-sum settlements and curtailments	11.6	26.6	0.0
Subtotal	29.1	44.9	16.3
Regulatory adjustment	19.0	3.9	31.8
Net pension cost	48.1	48.8	48.1
Spire Missouri | Other Postretirement Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	6.2	5.3	6.8
Interest cost on projected benefit obligation	4.5	4.7	6.9
Expected return on plan assets	(10.9)	(11.4)	(11.1)
Amortization of prior service (credit) cost	0.7	(0.2)	0.3
Amortization of actuarial (gain) loss	(1.5)	(2.0)	(0.5)
Subtotal	(1.0)	(3.6)	2.4
Regulatory adjustment	15.0	17.7	11.7
Net pension cost	14.0	14.1	14.1
Spire Alabama | Pension Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	5.5	6.1	6.2
Interest cost on projected benefit obligation	4.6	4.9	6.0
Expected return on plan assets	(5.8)	(6.9)	(7.3)
Amortization of prior service (credit) cost	(2.3)	(2.4)	(1.8)
Amortization of actuarial (gain) loss	3.9	3.1	0.8
Loss on lump-sum settlements and curtailments	6.6	5.0	0.0
Subtotal	12.5	9.8	3.9
Regulatory adjustment	0.7	1.8	6.9
Net pension cost	13.2	11.6	10.8
Spire Alabama | Other Postretirement Plans
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost – benefits earned during the period	0.9	0.4	0.4
Interest cost on projected benefit obligation	1.3	1.4	1.8
Expected return on plan assets	(4.9)	(5.0)	(4.8)
Amortization of prior service (credit) cost	0.3	(0.3)	(0.4)
Amortization of actuarial (gain) loss	0.0	0.0	0.0
Subtotal	(2.4)	(3.5)	(3.0)
Regulatory adjustment	(1.8)	(1.8)	(1.8)
Net pension cost	$ (4.2)	$ (5.3)	$ (4.8)